An American FBI agent named Steve Rossi, George Raft, slips past Heathrow's immigration controls, triggering a nationwide manhunt led by Scotland Yard. He makes his way into London and begins tracing a mysterious figure tied to aerospace work, starting with a barman named Bailey, Cyril Chamberlain, and asking about Michael Grand.
Bailey passes Rossi a note bearing an address that leads to a woman who claims she does not know Grand. She agrees to change Rossi's US currency and even buys him a coat while he waits in her flat, then, for money, provides another address: Kingston House, a swanky block on Kingston Road. He takes a taxi there and meets Joan Miller, Sally Gray, who acknowledges it is Grand's flat and says he is not there yet. She calls Grand, who agrees to meet Rossi, and Rossi contemplates staying in the flat, but Joan pulls a gun on him. The two drive to an old terraced house where Colonel Wilkes, Reginald Tate, begins quizzing Rossi. Rossi explains that Grand offered him a job working on aircraft design, and Joan is asked to accommodate Rossi in her own flat. Security Chief Brice, John Warwick, remains a looming presence as the investigation tightens.
Meanwhile the police post Rossi's picture in newspapers and question various underworld figures to locate him. The taxi driver who recognizes the image adds pressure to the case. A twist reveals that Joan Miller is an MI5 agent. Rossi then discloses that he is an FBI agent posing as a nuclear scientist, sent to infiltrate a gang that kidnaps Western scientists and moves them across the Iron Curtain. Grand has recently arranged the kidnapping of an American. With MI5's assistance, Rossi monitors Grand and arranges a meeting, but Grand and his organization grow increasingly suspicious and make several attempts to kill him.
Rossi and Joan follow Grand's assistant Irma Brookes, Patricia Laffan, and discover a body with some of Grand's ID in a burned-out car. Rossi is caught and taken to Hammersmith Police Station. It is explained that Rossi wanted to get into the papers so Grand would be aware of his presence. Rossi cannot identify the body in the mortuary as Grand, as he is unsure of Grand's appearance.
Grand is finally revealed when Max, Arnold Diamond, another taxi driver, informs him of what he has seen near the burning car and near the mortuary. Rossi dons a naval uniform and tails Grand on foot through empty city streets, leaving a chalk trail of markers for Joan to follow. Grand goes to the River Thames, where the low tide allows him to walk along the edge. They end up at the Old Swan Pier near London Bridge. Grand pulls a gun and a tense chase begins, leading to the rooftop of a grain elevator close to The Monument. They end up fighting on an external freight lift heading back to the ground. Rossi wins and the police arrive to bring the pursuit to a close.
