Set in London, the film follows the intertwining relationships of two couples, who spend much of their time together, both when working and in private.
Allen, the British producer of a successful TV show, is married to beautiful Carly, the star of his show. Ben is a young American, hired to write the script for their show, falls in love and marries sexy French girl Fiona, a make up artist for the show. They all live in London and spend much of their time - both at work and in private - together and it isn't long before they each have an affair with the partner of the other.
At 19:30, Ben dates Carly, Allen's wife, and they kiss on her doorstep, but they don't proceed to do anything more than that.
Ben marries Fiona. The two couples' friendship deepens and they hang out with each other.
At 40:25, Ben and Carly test if their partners will cheat on them. They set up this plan where the four of them will spend the weekend in a bed and breakfast hotel but when the weekend comes, they pretend to not be able to come and so only Allen and Fiona will be spending the weekend in the hotel. However, Ben and Carly are actually in the hotel as well, to monitor and see if the two will cheat. Long story short, Allen and Fiona cheat on them and have sex at the hotel while Ben and Carly listen to their moans in the next room. Only the pre-sex kissing between Allen and Fiona is shown.
The two couples break up and Allen marries Fiona. At 53:30, the newly married couple has sex.
This clip includes both couples's two main cheating scenes.