Amateur diver Henry Baker goes diving in the Caribbean, where he finds the wreck of a German U-boat. Inside he finds a briefcase which he takes back home with him. On his way home he meets his friend, diver Bob Carney, and lies to him about where he's been. When he opens the briefcase he finds a list of documents in German. His inability to read German frustrates him but one name leaps out at him: Martin Bormann. The dates on the diary appear strange to Baker as they seem to indicate that the submarine sailed after the war was over.

His next move is to go to England to see his friend, German speaker and Royal Navy man Garth Travers. Travers translates the documents and can't believe what he's found - information on Nazi sympathizers in the United Kingdom and America. The diary reveals that there's another case with the names still in the submarine. He gets in touch with an old friend of his, Brigadier Charles Ferguson, who works for the Prime Minister. The Prime Minister asks them to come to his office and invites two other men, Simon Carter and Sir Francis Pamer. After their meeting with the Prime Minister, they decide to ask Baker about the location of the submarine. Unfortunately for them, Baker died when he looked the wrong way and got run over by a London Bus.

Pamer discovers from his mother that his father had connections with the fascists during the Second World War. Pamer gets in touch with his Cuban friend Max Santiago for advice. Two men working for Santiago in London break into Travers house. Fortunately, the diary is not there because Ferguson has taken it. The men plant a bug on the phone before they leave (against the wishes of their boss Santiago). Travers' housekeeper comes home and finds the place in a mess. Travers contacts Ferguson. Ferguson is suspicious and asks his assistant inspector Jack Lane to check for bugs in the house.

Travers goes to the airport and picks up Jenny Grant, Baker's friend. They meet with Ferguson and she tells him what she knows but reveals that doesn't include the location of the submarine. Ferguson contacts Carter and Pamer and tells them what he knows. Pamer reveals this to Santiago and he promises to have the girl taken care of.

An IRA gunman and expert diver, Sean Dillon, is in prison in Yugoslavia. Ferguson and Lane visit him. Ferguson reveals to Dillon that he had him set up. Dillon had flown a plane carrying medical supplies to Yugoslavia but it also carried missiles, leading to Dillon being captured by the Serbs. Ferguson offers Dillon a clean slate if he works for them.

Dillon flies back to London with them. Ferguson takes Dillon to meet Simon Carter and Francis Pamer. Carter gets a fright when he finds out who Dillon is. Pamer passes on the information to Santiago. Dillon is with Jenny. Jenny goes for a walk. Jenny gets attacked by the two men working for Santiago but Dillon rescues her. Dillon wounds one of the men and he gives up Santiago's name. Jenny wants to flee to France and Dillon takes her to the airport. One of the men working for Santiago reveals to him that they gave us his name. Santiago orders his men to kill the two men. Dillon reveals to Travers and Ferguson that Jenny has gone to France. Ferguson isn't happy. Santiago flies to the Caribbean and meets up with one of his men, Algaro. He explains that they may have to deal with Dillon.