In London, Heather McMara, Bernadette O'Farrell, asks for help from an American writer, Philip 'Phil' O'Dell, to uncover who killed her brother Danny McMara, run over in a heavy nighttime fog. With no solid evidence to hand, the police are ready to do nothing. Undeterred, O'Dell and Heather follow a thread of clues that begins at Peggy's nightclub, a haunt Danny frequented, where they discover Danny's last known address.
O'Dell finds Danny's hotel room ransacked and is knocked unconscious by a man hiding inside, yet a tape recording survives. Among Danny’s letters is a note pointing O'Dell toward Marilyn Durant, an actress who has become despondent after a breakup with Danny. Marilyn hints at a producer, Christopher Hampden, who had arranged a screen test for her, but that line soon looks like a dead end. A speeding car nearly runs O'Dell down, and when his own quarters are ransacked again, he realizes someone is intent on stopping him. He also admits to Heather that he harbors romantic feelings for her.
Listening to the surviving tape, O'Dell hears a distraught man recount a garage fire at Gladstone, in which someone named George was killed. He tries to replay the confession for Heather, but accidentally erases the tape; still, enough detail remains to push the investigation forward. He learns that the man killed was George Maybrick, and that Hampden and Martin Sorrowby—treasured partners who moved to London with Mrs. Margaret Maybrick—were connected to the crime.
O'Dell confronts Hampden, who stonewalls but admits that Sorrowby, who had a wartime meltdown, ended up in a mental hospital at the time of a death two years earlier. Sneaking into the hospital, O'Dell confirms Sorrowby is still there and questions him. Sorrowby eventually admits that Margaret Maybrick engineered the murder, with Hampden and himself involved, but he also insists he is insane and cannot distinguish truth from fantasy. Sorrowby slips away, only to be killed soon after by Connors, Hampden's henchman.
Believing Danny may have been blackmailing Hampden, O'Dell shares his theory with Heather. A bombed building yields a fresh danger, and O'Dell is knocked out once again. Peggy—Margaret’s club owner wife, revealed to be the same Margaret Maybrick—appears and promises to bring Heather to Hampden. Peggy wraps herself in a distinctive scarf once worn by Danny's killer, intensifying the sense of danger. On another fog-drenched night, Peggy urges Heather to walk ahead with a flashlight, but the fog lifts just as the truth closes in. At Hampden’s residence, Heather learns Peggy’s true identity, and O'Dell arrives in time to rescue her.
The confrontation ends with Connors and Hampden dead, and Margaret presumed dead in a car crash as she tries to flee. In the aftermath, O'Dell and Heather leave London together for the United States, stepping into a new future as the mystery finally closes behind them.
