The work of Peter Ketley on Fisheries Research in Devon is being disorganised by a mysterious new method of salmon poaching. His young brother Bob, with the help of two children from London, eventually uncovers and catches the poachers.

The Cat Gang is a group of precocious British kids. They have a habit of hanging around a grown-up customs official who wishes that they'd beat it. But the Gang comes in handy when a smuggling gang arrives on the scene.