The film begins where a little girl, Sophie, is being bullied by other children. Only a bus driver and a boy, Julien, help her collect her books that the others have thrown into a puddle.
To cheer Sophie up, Julien gives her a small tin box, a gift from his fatally ill mother. Because it is important to him, he asks her to lend it back to him from time to time. As Julien wants the box back at the moment he gave it to her, Sophie demands proof of how important it is to him. Julien disengages the handbrake of the bus without hesitation, and the bus full of children rolls down a hill. Their game has begun: the box changes its owner after each completed dare.
Between the son of wealthy Belgian parents and the daughter of poor Polish immigrants a lasting friendship develops. As children, they misbehave in school, wreak havoc on a wedding, and request silly tasks of each other. As teenagers, their romantic relationships with others suffer as a result of their dares. Meanwhile, the two friends ignore any consequences or punishment during their game.
While they are always looking for the next kick, a love is slowly evolving between the protagonists. Not wanting to admit it, they divert their attention from it by even more extreme dares.
As young adults, Julien tells Sophie that he wants to get married, only later revealing that he means to someone else. The climax is reached when Sophie interrupts Julien’s wedding, after which he is cast out by his father and Sophie is nearly killed during another game. Julien returns to marry his wife, and Sophie declares that they will not see each other for ten years.
Ten years pass, and Julien is married with two children. Sophie has also married her husband, a famous soccer star. A successful Julien admits that he has not forgotten Sophie, though he assumes that she has forgotten him. On the night of Julien’s tenth wedding anniversary, Sophie sends a message to him, indicating that the game is back on. Julien and Sophie meet for a brief moment in the midst of another dare, yet it is enough to remind Julien that their game is “better than life itself.” After a dramatic accident, Julien and Sophie finally reunite, despite the protestations of their spouses.
The film has two alternate endings, which are shown consecutively. In the first, Julien and Sophie decide as an ultimate dare to finally share their dream together, their “dream of an eternal love” – the pair embrace while they stand in a construction pit that is about to be filled with concrete. The couple kiss as they are pulled beneath the cement, and both drown in the sludge. The other ending has the now aged Julien and Sophie spending time together in a garden and carrying on playing their game with milder dares. However, the opening scene of the film (an overhead view of a building site and a pit filled with concrete in which the upper side Julien’s tin box rests partially sunk) replays, suggesting that the two friends actually did bury themselves and drown beneath the concrete.