Having escaped the clutches of the criminal mastermind Mother and his life scrounging through the sewers of London, Joe Rat is back, now working as a sweeper in the more upscale Lomesbury Square. After he foils a burglary at one of the 'posh' houses on the square, the owner, Mr Harvey - a rich philanthropist - determines to help Joe. Harvey manages to track down Joe's father, a British army deserter now living wild in Canada. Joe has never met his father, but Harvey brings the wild man back to England and helps set them both up in better jobs. But when Harvey's son, jealous of the kindness shown to Joe, risks the lives of those around him, Joe will have to decide where his loyalties lie.