The film revolves around the intense relationship of the two teenage protagonists, Darren (Cillian Murphy) and Sinéad (Elaine Cassidy), who call each other "Pig" and "Runt," respectively. Pig and Runt were born at the same hospital at nearly the same time and grow up next door to each other. This brings about an extremely close relationship between the two that borders on telepathic. They live in their own world and barely interact with the one around them; when they do, it's mostly to express their hostility toward it. Until just before their 17th birthday, their relationship, while very intense and unhealthy, remains platonic.