Based on the graphic novel of the same name, Nick Sax is a corrupt, intoxicated, ex-cop turned hit man who is adrift in a twilight world of casual murder, soulless sex, and betrayal. After a hit goes wrong, Nick finds a bullet in his side, the cops and the mob on his tail, and a monstrous killer on the loose. But his world is about to be changed forever by a tiny, imaginary, blue-winged horse with a relentlessly positive attitude named Happy. On their journey, they must contend with a laundry list of enemies including angry mobsters, ex-mistresses, ex-wives, and one very bad Santa.
Sax and Happy embark on lonely journey in which Sax stumbles on a new plan to negotiate for his daughter's release; Happy takes solace in the welcoming arms of another Imaginary Friend.
Sax journeys through a dark past while he and Happy track his daughter; Merry and Amanda make a chilling discovery; Very Bad Santa coerces information out of Hailey about her dad.
Sax crashes a high-stakes poker game to win guns and cash for an escape from New York as Happy tries to convince him he's a father; ex-wife Amanda and ex-lover Merry team up to rescue Hailey.
A hit man must partner with an imaginary blue winged horse to rescue a missing child.