John Corby is birdwatching with his father Tom. Susan Fraser has found herself cut off from land so John helps her. Reaching land, she gives him a note to explain his absence to his father. However, nobody seems to know where Tom has gone.
John and Susan seek police help to look for Tom. The police won't intervene, as Tom Corby is suspended from Scotland Yard. Tom and his two contacts meet up with Kessler. Police surveillance believe Corby is on the take.
John and Susan catch sight of Tom but are unaware that a hitman is assigned to stop them. Tom wants the hitman stopped but the police are closing in. A motorcycle driver has been hired to cause an accident involving John.
John and Susan realise they're being tailed and outsmart their pursuer. When John gets too close to seeing his father, the pair are trapped underground.
Tom refuses to leave the Falcon pub unless he knows John is safe, but Bowers has conned him. With his father long gone, John and Susan reach an exit to the surface.
Susan finds a rope to help John but the motorcyclist is still in the area. A friendly garage attendant learns that the group holding John's father bought a map of Ullswater. John and Susan rent a canoe and attempt to outflank the abductors.
John and Susan are rescued from the lake. With Susan in hospital care, John learns his father is using an alias.
The motorcyclist forces John and Susan to ascend the mountain but he takes a bad fall. John leaves Susan to get help. The delirious biker gives a cryptic message to Susan.
Tom warns John about Bowers close by. Bowers takes Kessler to the airport to show him the target. John and Susan overhear Selby revealing information to his father.
Tom finds the man, who fit him up, is in charge so he asks for a deal. He knows Bartas is their target and wants in on the hit in Edinburgh.
John sees Kessler preparing his sniper's rifle, which puts the youngsters in grave danger. Tom is warned about asking questions. With the Tattoo drowning out the noise, John and Susan search the castle in search of Kessler's sniper nest.
Bartas is struck by a sniper's dart. Bowers pretends to be a doctor, pronounces cardiac arrest, and whisks him off in an ambulance. John goes in pursuit while Susan tells the police about the kidnapping. Tom realises Bowers wants him dead.
The police discover the stolen ambulance and close in. Bartas is moved but Tom makes a run for it before he's killed. He is thwarted by John's capture but by the time the gang reach the boat, the Coast Guard are on alert.