The film revolves around the intense relationship of the two teenage protagonists, Darren (Cillian Murphy) and Sinéad (Elaine Cassidy), nicknamed "Pig" and "Runt". Pig and Runt were born at the same hospital at nearly the same time and grow up right next door to each other. This brings about a very close relationship between the two that borders on telepathic. They live in their own world and barely communicate with the world around them. However, up until their seventeenth year, their relationship remains one of friendship, albeit a very intense unhealthy one.
Eventually, however, Runt catches and reciprocates the attentions of another young man, Marky (Darren Healy), from her school just as Pig begins to notice Runt. As their seventeenth birthdays draw closer, Pig's violent nature becomes more and more obvious and his romantic intentions towards Runt become confirmed when he kisses her after a rampage at a disco. Runt, however, does not want to give Pig what he wants of her. She does not know how to reject him; therefore, she continues her friendship, with him feeling awkward and caged.Pig remains very protective of Runt and finally their closeness raises the concerns of the school they attend. They are separated and Runt is sent away to boarding school. Pig is crushed by this and decides to track down Runt and make her his again. This starts off an irreversible chain of events that will ultimately end in tragedy. Pig finds Runt on their seventeenth birthday and takes her to a club. In a fit of jealous rage, however, Pig kills a boy who was dancing with Runt. The two flee the club and take a taxi to a beach where they make love. In the morning, Pig wordlessly allows Runt to smother him, knowing the punishment awaiting him. She stares out to sea considering suicide but decides against it.