Joanna and Michael (Keira Knightley and Sam Worthington) have been married for four years, having originally met in college.
At a party she gets to meet Laura (Eva Mendes), a new colleague of Michael's, and is suspicious when she sees the rapport between them, and later learns that she was one of the team away on a recent business trip with Michael, a fact he had not mentioned. At home later she accuses him of having an affair, which he firmly denies.
The next morning he is off on another business trip, again with Laura in the business party.
Joanna bumps into Alex (Guillaume Canet), who she had a relationship with some years earlier. It is clear that they both still have strong feelings for each other.
Laura is clearly out to seduce Michael - will she succeed? Or will Michael stay faithful to Joanna?
Alex is clearly out to seduce Joanna - will he succeed? Or will Joanna stay faithful to Michael?
This was emotionally exhausting, superbly acted, and utterly believable.