It is literally the last night, the world is ending, and humankind has known this for at least several months. Why it's ending is not really clear (it is mentioned off-hand that the sun is going supernova), it is just an accepted fact and not really dwelled upon.
The film follows several characters, none of them exceptional or heroic, just everyday people, and how they plan to spend their last hours. A man at the gas company calls all of his customers, thanking them for their business, wishing them luck, and calmly pledging to keep the gas service on as long as possible until the end. A young man is running through his checklist of all the sexual positions and situations he has always wanted to try, including having sex with his French teacher from his high school past. An older married couple invite their adult children over to pretend it is Christmas one last time, because that is when the family seemed happiest together.
The most interesting two characters are the ones whose paths overlap against their will: A woman (Sandra Oh) trying unsuccessfully to get across town in the last hours of chaos to be with her husband for a suicide pact, and a man (Don McKellar) whose wish was to spend the last night home alone with a bottle of wine. These characters provided some soul to a movie that otherwise felt somewhat detached, considering the content.