It’s said that nobody really understands a marriage except the man and woman in it. However, often they don’t truly know it, either. That’s just one of the lessons of “Last Night,” a gripping film about temptation, that is inexplicably opening at the arthouse, rather than a multiplex. Sure, it’s the directorial debut of a little-known screenwriter, Iranian-American Massy Tadjedin. But it has some fairly big names — Keira Knightley, Sam Worthington of “Avatar” — and a very big theme.

Joanna (Knightley) and Michael (Worthington) are a happily married couple. At least until a party brings Joanna in contact with Michael’s co-worker Laura (Eva Mendes). She spies the pair joking around, alone on the balcony. The wife is angry that her husband has mentioned the new hire, but hasn’t mentioned that she’s stunning. By the time Michael leaves the next morning for an overnight business trip, Joanna decides she’s overreacted. But whether it was going to happen anyway, or if his wife’s suspicions gave him the idea, the sparks fly that night between the two colleagues.