Bill Kirby, an insurance agent with a mysterious side business, tries to avoid a pair of suspicious-looking men whom he first observed on the Channel ferry from Le Havre. By scooping up a pretty young woman's luggage, he manages to elude them long enough to arrive at her marina, where he convinces her to take him across the Solent before they both come to harm at the strangers' hands.
Roger Fane complements Laura Marshall, his step-daughter, on the way she kept tabs on Bill Kirby after he left the Channel Ferry and asks her to maintain contact. He explains that he fears that Kirby has learned something vital that could be put to use to the detriment of the British government if he chooses to pass it on the wrong hands. Kirby takes a leave of absence from the his job as an insurance salesman. After failing to make contact with the intelligence community using a go-between, he discovers a friend on a rural road with a bullet in the back of his head.
Kirby obtains evidence that a key advisor to the British was a trained as Russian KGB agent and enters into negotiations with an American who implies he's working for the CIA. Kirby doesn't entirely trust the man, but agrees to secret rendezvous to exchange the information he's gathered. Meanwhile Roger Fane orders Laura to have no more dealings with Kirby, whom he regards as a potential traitor.
Kirby staggers home after being wounded at an intended rendezvous. He sends his son, Robin, to convince Laura not to return to school for the next term immediately and join him in an attempt to convince her stepfather that a dangerous traitor is serving as one of his key security advisers.
Kirby, Fane, Vincent and Laura travel to Normandy to listen to Madame Lafois' claims that Fane's American political adviser is actually an undercover Soviet agent, but learn that the old woman died after meeting with Kirby the previous weekend. Fane accuses Kirby of defaming Vincent and killing the French informant himself. The real traitors then force the rest of the party onto a remote beach with plans to kill them all.
