Joseph Rearden, a petty criminal-turned-agent for British intelligence, arrives in London. There, MI5 officer Mackintosh and his deputy, Mrs. Smith, brief him on a bold plan to steal diamonds that travel by post. Rearden carries out the scheme, even after punching a postman in the process. That evening, two detectives visit Rearden's hotel room, signaling trouble to come.
At his trial, the judge is infuriated by the failure to recover the diamonds and sentences [Joseph Rearden] to twenty years in jail. In prison, he gradually blends in with the other inmates and is assigned to laundry work. A few weeks in, an inmate mentions a clandestine organization that could spring him from custody in exchange for a share of the diamonds, and Rearden agrees to the deal. A staged diversion follows: smoke bombs explode over the walls, and Rearden, along with a fellow prisoner who turns out to be Slade, is hoisted out by a cargo net, drugged, and driven to a secluded location in the wild countryside. When he regains consciousness, he learns they will be kept there for a week while the manhunt subsides.
Back in London, the operation to draw out the escape network continues. Rearden’s entry into prison is a calculated sting meant to flush out the organization. In the House of Commons, an old friend and war comrade, Sir George Wheeler MP, delivers a heated speech attacking the way the Slade escape was handled. Mackintosh later counsels Wheeler to stay silent or risk public embarrassment. Wheeler, who hides his true intent behind a patriotic mask, is revealed as a Communist agent of the KGB who tips off the organization running the escape where Rearden is held. Mackintosh had suspected Wheeler and had used their meeting to try to flush him out, but before any action can be taken, Mackintosh is run down by a car and dies.
Meanwhile, Rearden comes under renewed suspicion from the escape organization. He is beaten and attacked by a guard dog, but he fights back, escapes the building, and sets it on fire. He is pursued by guards and the dog as he makes his way outside. To throw his pursuers off the trail, he drowns the dog in a nearby stream, then heads to a town on Ireland’s west coast, discovering that he has been staying at the estate of a friend of Wheeler. He contacts Mrs. Smith, who flies to Galway to find him, and the two plan their next move.
Realizing that Slade has been smuggled out of Ireland on Wheeler’s private yacht, Rearden and Mrs. Smith proceed to Valletta, Malta, where Wheeler is headed. In Malta, they attempt to infiltrate one of Wheeler’s gatherings and uncover Slade’s whereabouts. Wheeler recognizes Mrs. Smith—Mackintosh’s daughter—and drugs her, then takes her aboard his yacht. Rearden tries to enlist the Maltese police to raid the vessel, but they refuse to believe that Wheeler, a respected figure, could be involved in kidnapping and treason. Undeterred, Rearden is forced to pursue Wheeler as his own status makes arrest difficult.
On Wheeler’s yacht, the chase leads to a church where Rearden, Slade, and Mrs. Smith are together. Rearden draws his gun and orders the conspirators to surrender Mrs. Smith. Wheeler and Slade attempt to bargain for their safety, offering to spare Rearden in exchange for his cooperation. Reluctantly, Rearden agrees, but Mrs. Smith abruptly turns the tables, firing and killing both Slade and Wheeler, in effect avenging Mackintosh’s murder. Afterward, she cuts ties with Rearden, angry that he did not follow his own orders.
