As Alan’s leave in Sussex comes to an end, he and Lorna become engaged, and he sells his motorbike to buy her a ring. Meanwhile, Harry leaves Becket’s Hill to work in a munitions factory. Triggers is frustrated by the attitude of his commanding officer, until the Major, burdened by the responsibility of his position, takes over Triggers' reconnaissance duties and is shot down and killed. Enraged by the RFC losses against superior German machines, Triggers remembers one of the Major's comments and attempts to ram an Eindecker, but fails to bring it down. On the same day, a general arrives to visit the base, and meets Alan as the latter returns from leave. Triggers' frequent protests to senior commanders at last bear fruit, as the general orders ‘C’ Flight into forward action, with the task of keeping the Eindeckers busy so as to prevent them from shooting down British reconnaissance planes. For the first time in the series, German pilots are shown discussing tactics. All the observers except Bravington are sent away to join the reconnaissance effort, and Triggers introduces a new regime, with Sergeant Mills instructing the pilots in keep-fit exercises. With a new French engine and a Lewis gun fitted to the BE2, Alan is sent to try out a new approach and succeeds in unnerving the German pilots.

With German Zeppelins making bombing raids across Britain, Major Lancing arrives with orders for ‘C’ Flight to forget the Eindeckers and bomb the Zeppelin sheds in Belgium. The pilots are instructed in bombing techniques by the experienced Sgt Hollywake but his disregard for the threat of the German monoplanes angers Alan, who is becoming obsessed with the Eindecker. At home, the threat of the Zeppelins is giving concern, but Molly is even more worried by Tom's inability to cope on his own in the smithy, and asks him to train her as his assistant. Triggers, Mills and Bravington eventually develop a plan to intercept a Zeppelin on its way home. Too late to impress a conference on the flying service, the last bombing raid is a failure on all counts: not only does Hollywake offload his bombs to avoid engaging superior numbers, but Alan’s BE2 is shot down behind enemy lines. Just as Molly is congratulating herself on her first horseshoe, a telegram arrives with the news.

The replacement pilot arrives in the form of Lieutenant Michael Starling, an Oxford academic keen to show off his expertise in flying theory. Meanwhile Triggers is ordered to drop a British spy behind enemy lines and is shocked when the spy turns out to be an attractive young widow, Madame Boissier. When their plane suffers engine failure, they are forced to land and take refuge with a French priest. Back at base, Gaylion, now acting flight commander, refuses to order a rescue mission. While Triggers awaits the arrival of a local mechanic, Starling locates the grounded plane and destroys it, cutting off their escape route. Triggers, now out of uniform in the hope of escaping detection, and Madame Boissier are betrayed to the Germans by the mechanic. Despite the efforts of a sympathetic German defending officer, they are both found guilty of spying and sentenced to death by firing squad at dawn the following day. Madame Boissier attempts to save Triggers by revealing to the court that he is a British officer, but both are imprisoned overnight. Triggers confides in Madame Boissier about his troubled family life, and begins to feel affection for her. The Germans, having found his uniform and papers at the priest's house, make him a prisoner of war but no reprieve is forthcoming for Madame Boissier, who is shot while Triggers watches from a window. Incensed by her treatment, he escapes from the prisoner transport and returns to base, picks up a plane and strafes the car carrying the German commanding officer away from the place of execution. Only to Starling does he reveal something of his true feelings.

When Charles Gaylion crashes a BE2 on takeoff, he fears losing his nerve. He attempts to take another plane up at once but there is an accident and one of the mechanics loses both of his hands. Despite the fact that a faulty ignition switch is discovered, Gaylion blames himself and begins to behave increasingly erratically. Bravington attempts to cover up for him but the two end up fighting in the mess. In the meantime, Triggers, having recognised the signs of stress in his most experienced pilot, is attempting to get him temporarily posted back to Britain as a flying instructor. The transfer comes through on Charles's birthday, and his fellow officers arrange an impromptu party; on hearing the news, Charles bursts into tears of relief. Returning to his family home in London, he finds that his sister Kate has become an ambulance driver and his parents are expecting a government minister to dinner. His father, now promoted to major-general, assumes that Charles has taken his new posting unwillingly and would prefer to be at the Front. Although slightly drunk, Charles gives the minister his informed opinions on the shortcomings of the BE2 over dinner, and his father is surprised to find that his forthright speech has made a favourable impression on their guest. However, after the minister has left, an argument breaks out and a hysterical Charles admits the reason for his transfer, incurring his father's displeasure. Only Kate appears to feel some sympathy for his predicament.

Charles Gaylion, sent to train new pilots in the south of England, quickly becomes frustrated at the rules that forbid him teaching the trainees anything potentially dangerous. His ideas for a new training system are dismissed as nonsense by his commanding officer, who will not allow new pilots to fly, alone or accompanied, in the slightest adverse weather conditions. Charles does, however, find a home from home at Becket’s Hill, where he begins to visit Molly and Lorna on a regular basis. Molly still has some hope of Alan being found alive, but Lorna feels sure that she will never see him again. After an argument with one angry flying pupil, Lt Towy-Jones, who questions his courage, Charles demonstrates how to get out of a spin, allowing the trainee to try it for himself even though it means risking both their lives. However, a less able pupil, Westerley, hearing about his friend's success, decides to take a plane up alone and attempt the same manoeuvre, while Charles is off the base taking Lorna up for a pleasure flight. Westerley crashes and is killed, but, when Charles attempts to take responsibility for the incident, he receives unexpected sympathy from his CO and is not blamed. At the end of the episode, the viewer catches a glimpse of Alan Farmer, semi-conscious and being cared for in a private house.

Alan is being cared for by a French family close to a German airfield. Gradually recovering from his accident, with the help of the daughter of the house, Françoise, he conceives a plan of escape. Just as C Flight are being congratulated by the general on the success of their "forward action", a German attack on Sainte Marie takes them by surprise. All but one of the planes are destroyed, and several men are killed. Bravington, frustrated at no longer being able to take an active part in missions, quarrels with Triggers over how to retaliate; Triggers forces the remaining men to work all night to repair the one surviving BE2. Disguised as a peasant bringing the early morning milk delivery, Alan gains entry to the German airfield and manages to make his escape in an Eindecker. However, lying in wait for him is Captain Triggers in the last BE2, intent on taking his revenge for the attack on Sainte Marie. Having forced down the German plane, he is about to shoot Alan when they recognise one another. Back in England, Molly has become resentful of the amount of time Lorna is spending with Charles, and she quarrels with both of them over their apparent betrayal of Alan; Lorna herself is convinced that Alan is dead. Charles takes Lorna to meet his family, but only his sister Kate is at home. When they are left alone, Kate forces Charles to admit that he is in love with Lorna. Triggers sends Alan home on leave, in the expectation that his wedding to Lorna will take place. However, the news of Alan's safe return causes Charles and Lorna to acknowledge their feelings for one another, and Lorna tells Charles she cannot marry Alan.

Bravington, feeling under-employed, is determined to train as a pilot, and makes an unauthorised solo flight, much to Triggers' annoyance. Triggers is ordered to deal with a German machine-gun post prior to a big "push", but the duo's attempts to attack it from the air prove ineffectual. Alan returns to Sussex on leave. When Charles tells Lorna of his intention to "bow out", she confides in her mother that she no longer wants to marry Alan. Mrs Collins, seeing Charles as a better "catch", is cool towards Alan, when he visits her to talk about plans for the wedding. Shortly afterwards, he sees Charles and Lorna kissing, and retreats without being seen. Unaware that Charles has already arranged to be posted back to France, Alan goes to the training base and confronts him. Charles accepts the blame for his relationship with Lorna, but Alan will not be placated and gives him a beating. With his plan to bomb the German post having failed, Triggers decides to land behind enemy lines and attack it with grenades. On landing, he is met by a German general, who is captured and taken back to Sainte Marie by Bravington, on Triggers' orders. Triggers sets off to attack the post alone, telling Bravington to send Starling back for him, but Starling is out on patrol and Bravington has to go back himself. Triggers manages to disable the machine-gun post, and steals the general's car to escape from the pursuing Germans. After Bravington successfully flies them both back to base, Triggers agrees that he has proved his suitability for pilot training, but cannot recommend him because of a new directive preventing observers from retraining as pilots. Alan visits Lorna and realises that the feelings she and Charles have developed for one another are genuine. Lorna announces her intention to go to France as a nurse. Alan apologises for his behaviour and tells his mother that he will take over from her at the smithy for the remainder of his leave. This is the last episode to feature Alan's

Alan Farmer shoots down a German observation balloon, but spares the unarmed observer; he is told by Triggers to stay away from the balloons in future. Triggers is intending to recommend Farmer for a commission, but is disappointed with the response from his other officers. Bravington has reservations about his "suitability". Mills and Starling accept the news philosophically but there are signs of resentment from the NCOs. When a German plane drops a note about an injured British airman, Alan misunderstands his purpose and shoots down the German, gaining further criticism. After a disastrous interview, in which Alan’s background and academic record are picked over by the selection panel, he goes to a bar where he experiences further resentment from embittered members of an infantry division. Alan drinks with their sergeant, who is at first sympathetic, but then criticises Alan for not having killed the German observer, telling him that the balloon was quickly repaired. That same evening, Triggers is visited by the chairman of the selection panel, who wants to understand his reasons for recommending Alan for a commission. On Alan's next flight, he makes a point of attacking the German observation balloon again, this time killing the observer; to his surprise, Triggers seems pleased by his action. He then receives the news that he has been commissioned, and is welcomed into the officers' mess.

Triggers arranges for Alan to share Charles Gaylion's quarters, hoping that the more experienced officer will help him cope with the insubordination he still experiences, following his promotion. Both are initially unhappy with the move, and quarrel when Alan discovers that Lorna has written to Charles. The "guardian angel" parachute remains unused by the RFC because of its excessive weight and awkwardness. An American engineer, Leroy Schultz, has invented an alternative parachute which can be opened manually but can only be used once. "Special Flight" are asked by the general to see Schultz and give their views; Alan is broadly in favour of anything that could save pilots' lives, but Triggers fears it will encourage the pilot to jump and sees it as an excuse for cowardice. Schultz, previously the proprietor of a "flying circus", stands to make a fortune should the RFC take the parachute, and is regarded with suspicion, particularly after he offers Alan a cut of the profits in return for his support. Alan is prepared to volunteer for a demonstration jump, but Triggers will not allow any of his men to make the jump. After Schultz confesses to Alan that he has not used the parachute himself since a fatal accident involving a member of the public, Alan persuades him to do the jump. Just before take-off, an inquisitive airman, left alone in Triggers' office, plays with the parachute and clumsily attempts to conceal what he has done. When Schultz realises the chute has been tampered with and he has no time to re-pack it, he agrees to jump, but loses his nerve when Alan takes him up in the plane. The idea of the parachute is abandoned, but Charles admires Alan's courage

As C Flight puzzle over how to protect reconnaissance aircraft from the Germans, Triggers is astonished to be recalled to Britain. He finds he has been seconded as an advisor and test pilot at the factory which is to build a new fighter - the factory owned by Triggers’ own father. The initial signs are good: the new "Viper" has a better rate of climb and will be equipped with a forward-mounted machine gun, but Triggers has concerns. His relationship with his father is fraught, and he is annoyed that Mr Triggers wants to press ahead with manufacturing the plane in bulk before resolving issues such as the purchase of a suitable engine from Spain. Following a test flight, he insists the design be amended before putting the prototype into production to make it safe enough for the average pilot to fly. In an attempt to prove that the "Viper" can be handled by the average pilot, the designer Tony Snow takes it for a further test flight. The plane crashes in flames, and Snow is killed. Triggers' father still wants him to remain at home to take charge of the factory he will one day inherit. Triggers, who is uninterested in the business, applies to return to France and is posted back to C Flight where his pilots are still unsuccessfully trying to shoot down the enemy's Eindeckers.

Soldiers from a battalion stationed nearby break into Sainte Marie and steal the pilots’ rations. Led by a former university lecturer, they have mutinied because of harsh and unfair treatment; their CO attempts unsuccessfully to impose discipline. When an enraged Triggers threatens to take the law into his own hands, he is encouraged by the response of the army authorities, and decides to take two planes to the mutineers' encampment and frighten them into submission. Bravington opposes the plan. When Starling finds out what is proposed, he secretly visits the mutineers’ encampment in an attempt to reason with them, but they refuse to allow him to leave. When the planes from ‘C’ Flight set out to rescue Starling, Triggers instructs Alan to shoot anyone who tries to prevent Starling leaving. Alan fires the first shot in a gunfight which results in Starling being wounded and all the mutineers being killed. In order to cover up the truth about the incident, Alan and Starling are both reprimanded by Triggers. While visiting Starling in hospital, Alan and Charles meet Lorna again, but she angrily rejects advances from both of them.

The pilots of ‘C’ Flight are dispatched to Veuve-sur-Meuse under the command of Captain Boucharlat to assist in fighting a surprisingly large number of Eindeckers. Gaylion is delayed on take-off and is shot down by an Eindecker. Boucharlat shows a lack of sensitivity and Alan reacts violently to his apparent lack of concern over the missing pilot. Having destroyed his aircraft, Gaylion is befriended by a Frenchman who knew his uncle at The Hague. He informs Gaylion of the Germans’ plans for a massive offensive, something Triggers and Starling have begun to suspect from the Germans’ movements. Boucharlat refuses to listen and irritates the pilots of ‘C’ Flight, especially Starling, with his mocking attitude towards the British equipment and their lack of medals. Their housekeeper’s 16-year-old daughter grows attached to Triggers, who finds her attentions amusing and takes an interest in her collection of souvenirs. When he agrees to give her one of his spare buttons, the girl responds by kissing him, but her mother enters the room at that moment, jumps to the wrong conclusion and locks her in her room. Boucharlat accuses Triggers of attempting to seduce the girl. Charles Gaylion's French acquaintance is killed, but Charles makes it back across the lines with the intelligence; Starling is surprised when Boucharlat confesses he has suspected the truth all along. In the meantime, angered by Boucharlat’s attitude, Triggers and Alan take to the air, determined to show that they can shoot down an Eindecker. They succeed, but the plane crashes into the church, killing several civilians including the housekeeper. The British pilots are obliged to leave the area.

Lorna visits Sainte Marie to look for Charles Gaylion, and tells him she wants Alan to stop visiting her; Charles impulsively asks her to marry him, and she accepts. Dick Bravington is aware of her visit, but neither he nor Charles tells Alan about it. Michael Starling goes out on patrol and is faced with two Eindeckers working as a team. They shoot him down and his plane continues to circle eerily for some time after he is dead; the loss makes an impact on the rest of the officers. With a big 'push' due, Colonel Smith arrives with orders that ‘C’ Flight are to adopt new signalling methods and patrol the lines, reporting progress. Smith insists on accompanying Triggers on his patrol to view battle conditions, and Triggers takes a delight in making sure he experiences the worst aspects of such a patrol. Their plane is hit and forced down in No Man’s Land. Taking cover in a ruined building, they find two trapped soldiers and realise they are under attack from both sides of the lines. Triggers and the men make it to a bomb crater but Smith is killed attempting to reach the British lines. Despite Triggers' instructions that planes should continue to fly alone, Alan insists on accompanying Charles on patrol. They succeed in shooting down an Eindecker, but Bravington is unimpressed, and Farmer accuses him of resentment at the fact that Farmer has been promoted whilst Bravington is unable to get pilot training. When he confronts Charles about what he sees as his and Dick's poor treatment of him, Charles confesses that he is engaged to Lorna. On hearing that Triggers has been shot down, the two pilots take off in an attempt to draw the German fire to allow Triggers and the others to return to their own trenches, but Triggers is hit by a shell blast while trying to carry a fatally-wounded man to safety. 'C' Flight are assigned to a new squadron, and the three remaining officers commiserate with one another on having to leave their base and not having Triggers around an