He is investigating a leak of sensitive material, critical to his companies financial future, and all fingers soon point to Pelham himself (a clandestine meeting on a paddle boat on the Serpentine is ludicrously brilliant). This espionage sub-plot plays well with the Pelham's mounting paranoia, and the gradual loss of his faculties.
Moore is brilliant in the dual role, (but in fairness, he is a favourite of mine and yes, he's my Bond) and the rest of the cast, which includes a few recognisable faces, are equally adept. When he bumps (literally) into a beautiful fashion photographer at the swimming pool, he even manages to drop in some shutterbug jargon based innuendo.
It's directed with gusto by Basil Dearden, (who was responsible for both the linking and hearse stories in the Ealing classic Dead of Night (1945) and wrote several George Formby films), from a story by Anthony Armstrong. It had previously been adapted for TV for an Alfred Hitchcock Presents episode, and it is to the credit of Dearden (and no doubt uncredited screenwriter/producer Bryan Forbes) that it doesn't feel padded. It is naturally paced, and not short of shock moments, while never going over the top. The effects with the dual characters is remarkably well done also.
8 out of 10