The movie Last Night is an interesting and realistic depiction of modern marriage and explores the themes of trust, infidelity, and the fact that we all have a past. The movie centers around married couple Michael and Joanna, played by Sam Worthington and Keira Knightley, respectively. Michael and Joanna are by all accounts a relatively happily married couple. They have some issues and are far from perfect, but it's clear that they love each other and are happy being married to one another. They have a small fight but quickly make up on the eve that Michael has to leave town for a night on business, leaving Joanna alone for the night. It is on this night that both become tempted by the potential of infidelity.
Michael is tempted by a woman co-worker that is on his business trip, Laura, played by Eva Mendes. Michael and Laura end up staying out late together, after the rest of the group on the business trip has returned to their hotel room. Laura makes it well known that she is attracted to Michael and would be more than willing to carry on an affair with him, or even a one-night stand. This leaves the ball in Michael's court as to whether or not he would like to follow through with the affair.
Meanwhile, while out Joanna bumps into a former boyfriend of hers, Alex, played by Guallaume Canet. Joanna and Alex clearly have some unfinished business in their relationship, with Joanna explaining to him that he may not necessarily be the wrong person for her, he may have just come around at the wrong time. The two plan an evening together to catch up, which turns into a rather adventurous night involving a party, a dog, and lost keys. When things settle down, the opportunity for Joanna to have an intimate encounter with her former flame is clearly on the table.
In the end, Joanna and Michael are re-united the next day, and they must face each other with whatever they did or didn't do the previous night.