Harry Lockhart is whisked from a life of petty crime to Hollywood where he'll audition for the role of a movie detective and be tutored for the part by a private eye. Now all he has to do is convince the dream girl he meets that he's a real detective and try not to stumble across the corpses that start to appear as he gets involved with real murders.

Petty thief Harry Lockhart (Robert Downey Jr.) stumbles into an actors audition whilst on the run and before he knows what's happening is whisked off to Hollywood for a screen test. Harry is teamed with ruthless, relentlessly tough and gay private eye Perry van Shrike (Val Kilmer), nicknamed Gay Perry to prepare for his film role.

Then Harry meets his childhood sweetheart, Harmony (Michelle Monaghan), a failing actress working the party scene. And then the bodies start turning up and what began as a free trip to L.A. may result in Harrys one-way ticket to the city morgue.