Siblings Helen and Dan make a telescope and go to the local observatory to show their friend Tom. Mr Bunting, one of the observatory employees is impressed by Helen and Dan's telescope. Dan takes the time while at the observatory to note down the registration number on Tom's car as it is a hobby of his. The children go into the observatory with Mr Bunting. He shows them a meteorite. Later that night the children are looking at the stars through their telescope when they see a bright light falling from the sky.

The bright light that the children saw falls into some nearby trees and the following morning the childen go to the woods to investigate. When they are in the woods it is eerily quiet and Tom's compass spins round and round so they don't know which is the right direction to go in. The children are scared and run out of the woods. Once they are out of the woods they reach a sandpit and Tom finds that his compass works again. The children hear a strange noise and see a tall thin man in a familiar looking car appear. The children are once again scared and run away from the man, but Tom trips and the man approaches

The children manage to escape from the man but Dan goes back to look at the registration number on the man's car. He realises that the registration number is the same as the one he noted down for Tom's car. As the children are about to leave the sandpit they see a boy approaching wearing a blue and silver suit.

The children think that the boy in the blue and silver suit is from space. They try to talk to the boy, who they call peep-peep, but they are not sure that he understands them. Helen goes to tell Mr Bunting about peep-peep but Mr Bunting doesn't believe her. The children take peep-peep to Mr Bunting who shows the boy a star chart. The children and Mr Bunting find out that peep-peep's spaceship was collecting meteorites from Mars before arriving on Earth. Peep-peep tries to write something for the children but they can't read it. Peep-peep collapses.

Mr Bunting takes Peep-Peep to the hospital while Helen and Dan wait at the observatory to wait for Tom. On the way to the hospital, The Thin Man fires at Mr Bunting's car causing the engine to stop. Back at the observatory, Helen realises that she can read Peep-Peep's message in a reflection.

Helen and Dan get worried when they call the hospital and are informed that Mr. Bunting and Peep-Peep did not arrive. Meanwhile, The Thin Man pulls up behind Mr. Bunting's broken down car, arriving in Tom's car, and points a gun at Peep-Peep and Mr. Bunting. The pair are terrified as The Thin Man makes them go into the woods. Back at the observatory, when Helen tries to call the police she discovers the phone line has been disconnected. Dan and Helen start to panic when they hear someone approching via the stairs.

The person that Dan and Helen heard coming up the stairs in the previous episode turns out to be Tom. Tom and the children head out to look for Mr Bunting. Meanwhile, Mr Bunting and Peep-peep are trapped inside a spaceship by The Thin Man. With them are lots of meteorites and there is also another spaceman locked with them. Mr Bunting believes this is Peep-Peep's father. Peep-Peep's father writes Mr Bunting a note, but Mr Bunting is unable to read it. Out on the road, Tom and the children find some tyre tracks. They find Mr Bunting's book with a message from Peep-Peep telling the children that Mr Bunting and Peep Peep have been captured.

Tom and the children decide to find the spaceship before calling the police. They head for the sandpit and go from there, but they cannot find the ship. They come out of the woods at a lake, and the compass keeps pointing into the middle of the lake. They realise that the ship must be under the water. In the meantime, The Thin Man is trying to get Peep-Peep's father to start the spaceship, but Peep-Peep's father will not, so The Thin Man keeps trying to start it himself.

Tom and the children look for a way into the spaceship but they are being watched by The Thin Man. Tom is captured so Dan and Helen run away. The Thin Man manages to capture Dan, but Helen helps him to escape by using the mirror she is carrying to shine the sun into The Thin Man's eyes. As Helen is distracting The Thin Man, Dan sends himself into the spaceship to save the others.

Helen is captured by the thin man and Dan must save her but he can't hear what everyone is saying behind the spaceship's glass. Peep-Peep comes to the rescue by writing a message on the glass to Dan, who manages to escape in the nick of time. Later, the kids learn how Peep-Peep learned to spell backwards & send him home with a parting gift of Dan's compass.