Then they meet a man named Robert and his wife, Caroline, who is crippled. Colin and Mary seem happy for the diversion--happy to meet another couple that takes the focus of off them (off of each other) for a while. Things become strange (and stranger yet; one could say horrific) when they attempt to leave: Robert and Caroline insist that they stay with them for a while longer.
While Mary and Colin indeed rediscover each other in ways during this time--an erotic attraction to each other that was below the surface--they also find that their relationship/friendship with Robert and Caroline takes turns that are likewise erotic and violent in nature. A pervasive dread runs through this novel, leading to the terrible climax that no reader could predict. Absolutely in the key of McEwan, without match in the genre, and a very worthwhile read.
From that moment, Lewis spirals into bereavement that has effects on his relationship with his wife, his psyche, and with time itself: "It was a wonder there could be so much movement, so much purpose, all the time. He himself had none."