The MacKintosh Man (1973)

Joseph Rearden, a British Intelligence agent, arrives in London and makes a rendezvous with Mackintosh, the head of his organisation, in a discreet office located just off Trafalgar Square. Mackintosh and his deputy, Mrs Smith, inform him of a simple way to steal diamonds which are transported via the postal service to avoid attention. This he does, apparently getting successfully away after punching a postman, and making off with the diamond-filled parcel. However, that evening, in his hotel room he is paid a visit by two Metropolitan Police detectives who have received an anonymous phone call advising them about the robbery. They are unconvinced by Rearden’s pretence to be an innocent Australian who had recently arrived in London.

The judge at his trial is angered by the failure to recover the stolen diamonds from Rearden, who he believes has stashed them away somewhere, and sentences him to twenty years in jail. Rearden is shipped off to HM Prison Chelmsford. He slowly begins to blend in with the other prisoners, and is assigned to laundry-washing duties. A few days after entering he encounters Slade, a former British intelligence officer kept in high security after having been exposed as a KGB mole. He makes innocent enquiries of his fellow inmates about Slade, but not a great deal is known about him.