Mary and Colin have a seven-yearlong relationship, but live apart - Mary also has two kids from a previous marriage. They always spend the summer holidays together, and their vacation has now brought them to a city that may or may not be Venice (McEwan probably keeps it vague because he has changed some elements to suit the story). Mary and Colin have grown a bit weary of each other, even their sex has become a mannerism.
One evening, looking for something to eat after all restaurants have already closed, they get lost in the endlessly winding streets and alleys of the ancient city and are picked up by an enigmatic Italian called Robert. Robert has an open shirt, revealing his hairy chest and a gilded razor blade hanging around his neck. He is forceful and insistent, and can't keep his hands off the handsome but weak Colin. About women he has a rather chauvinistic and patriarchal view. He guides them to a bar he owns for a bite and a bottle of wine - the other customers seem all homosexual men.
The next morning he takes them to his apartment, where they also meet his Canadian wife, Caroline, an invalid. Although they are shown great hospitality, Caroline has spied on them while they were sleeping naked; and later, before dinner, Robert will punch Colin playfully - but very painfully - in the stomach.
It appears that Robert is the product of a sadistic upbringing (he tells some weird tales about his domineering father's strictness and his bitterly jealous sisters), while Caroline has an uncomfortable masochistic view of life - she sees men as masters to whom women should yield. Is she an invalid because something went wrong during an SM session with Robert? When Mary and Colin leave to go back to the hotel, they hear a slapping sound behind the just closed door, as if Robert has hit his wife, but they can't be sure about that.
By now, every reader can smell disaster - of course, Mary and Colin should never go back to Robert and Caroline again. But they feel strangely inspired by the meeting and back in the hotel, they make love, sleep, and make love again - for days on end. They also have sadomasochistic dreams...
They have been hypnotized, as it were, by the mysterious other couple, and are like rabbits, sitting still, while the snake bends over them... There is a huge thrill at the end, but I will say no more.
McEwan tells this terrifying story of sadomasochism and ritualized murder in his usual cool and precise style.