Having escaped the clutches of Mother in Poundsfield, Joe Rat is back, now working as a sweeper in the more upmarket Lomesbury Square. Having foiled 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. However, Alec Harvey has other ideas - he's jealous of the kindness his father shows Joe and the relationship Joe seems to have with this exciting man from Canada. He determines to show his father once and for all that he's just as good, but in so doing risks the lives of those around him.