It's about a little boy's struggle with his mother's heroin addiction. Keira plays a young waitress who befriends the boy, but has a herion problem of her own. The boy's mother straightens out, but Keira's character winds up as a prostitute, turning tricks to pay for her habit.
"Purgatory's kind of like the in-betweeny one. You weren't really shit, but you weren't all that great either. Like Tottenham."
I'll try being nicer...if you'll try being smarter.