A photographer discovers a murder in the background of a candid photo.

Thomas, a successful London photographer, spends a night in a flophouse, photographing derelicts for the book he is preparing. The following day he photographs a fashion layout and then speeds off in his Rolls Royce to inspect an antique shop he intends to buy. He strolls into a nearby park and catches sight of a couple playfully making love. Amused, he proceeds to take pictures of their apparent bliss until the woman, Jane, sees him and asks for the roll of film. He refuses, and later she traces him to his studio and flatly offers herself in exchange for the negatives. After tricking her into accepting a different roll of film, Thomas develops his pictures. An expression on Jane's face as she appears to be glancing toward some bushes arouses his curiosity, and he starts making blowups of the frames of film. Thomas feverishly enlarges each photo in sequence until suddenly he sees the shadowy figure of a man with a gun lurking in the bushes. The paradoxical nature of his discovery--the difference between what he thought he saw and what actually took place--irritates and confuses him, as does his sudden passion to know the truth. In an interlude, he indulges himself with two teenaged would-be models who visit his studio, but, inevitably, he is drawn back to the park, and he finds the dead body of the man he photographed with Jane. He races back to his studio, but the photographs and negatives have been stolen. When he returns to the park again there is no longer a body. All he sees is a group of mimes playing an imaginary game of tennis. He joins in their game for a moment by tossing an invisible ball back to them, and then he walks slowly away.