A man (Eckhart) offers a bridesmaid (Bonham Carter) a glass of champagne at the start of a wedding reception. She responds that she doesn't drink anymore but still enjoys a cigarette. They begin to flirt, and a conversation begins. Among other things, the couple discuss past relationships, their mutual proximity to age 40, the differences between lawyers and doctors. The man talks about certain memories as if they were common to the two of them.

Despite revelations of the man's current girlfriend, 22-year-old Sarah the dancer, and the woman's husband, 45-year-old Jeffrey the cardiologist, the conversation heats up and they end up in the bridesmaid's hotel room, where the close encounter gets even closer. The woman reminds the man that her plane, traveling back to London, leaves at 6:00 a.m. Their mutual memories and disclosures continue to indicate that these two have met before.

The two leave the hotel together, but enter separate taxicabs. The film ends in ambiguity, as the man and woman, apparently in different places but sharing one frame, speculate with their driver(s) on the future and the difficulty of being happy.

source: wikipedia.org