Found living on the streets as a child, Thomas has no memory of his childhood. Now grown, he has his own family, but his son, David, has leukaemia.
One night, he hits a homeless man, Mack, with his car. He tells Tom he was once part of London's underground homeless and his son's leukaemia is not all it seems.
To save his son's life, he must return underground and face the darkness waiting for him.