Sue, an art student, and her boyfriend Dave, a mechanic, inadvertently stumble onto a plot by two villains, Naylor and Crouch, to free a dissident scientist from prison.

Naylor and Crouch use scientist Hassell's knowledge to steal top-secret technology, Libra, from a military base. They blackmail Dave's dad, Phil, a reformed criminal, into helping with the plot.

Sue attends an art auction with Prof. Rumford. Naylor and Crouch disable the closed-circuit TV system at the National Gallery of Art. Sue and Dave track them to their hideout, but are caught.

Phil installs a mysterious device in 16 vans. Sue, Dave, Naylor, Crouch, Rumsford, and the vans travel by boat to France. Naylor and Crouch drive to a large chateau owned by Goddard; Sue and Dave follow, hitchhiking.

Sue and Dave enter the chateau with the help of Jacques, a former resistance fighter. They find painters copying old masters. Naylor and Crouch chase them towards the river, where they take refuge with a canoeing tour group.

Naylor and Crouch pursue Sue and Dave, but Jacques disables their car. Goddard reveals the purpose of Libra. Sue, Dave, and Jacques follow one of the vans to a fine art museum, and see Libra put to use.

Sue is caught and taken back to the chateau. Goddard reveals his true intentions to Rumsford, who is then locked up. 15 vans return bearing masterpieces stolen from museums and replaced with fakes. Sue and Rumsford escape.

The stolen paintings are stored in a secret room with a remote-control bomb. Goddard tells Naylor that he never intended to sell them. Jacques sets up a meeting with Goddard; Naylor and Crouch disarm the bomb and steal back the paintings.

Naylor and Crouch hole up in a crypt. Cunliffe, a military intelligence officer, arrives and tells Sue and Dave to go home. Pretending to be a buyer for the paintings and Libra, Cunliffe contacts Crouch. Sue and Dave's car breaks down.

Jacques, Rumford, and Cunliffe trick Naylor and Crouch into giving them the paintings, but the pair escape with Libra. Sue and Dave follow. They meet another buyer, Hugh, who spirits them off to England in a helicopter with captive Dave.

Hugh, Naylor, and Crouch use Libra to briefly shut down a major power station in order to tap into its network. Hugh blackmails Phil and Dave's brother, Bob, an ex-Army engineer, into assisting them.

Dave escapes. Naylor and Crouch, followed by Sue and Dave, attend a local fair intending to steal an antique steam tractor. Cunliffe and Sue figure out Hugh's plan to steal secret technology from a covert base.

Naylor and Crouch proceed to the base in the electronics-proof tractor, but are stopped by Sue, Dave, Bob, and Cunliffe. When Hugh's men arrive they are met by police, the plot foiled. Naylor and Crouch escape and wander off, bickering.