The United Kingdom is free of rabies, but the danger is ever-present.
Billy Harsley is eleven and hyperactive. School bores him and he longs to have adventures like Huck Finn. He has run away for the third time and is sleeping rough on the marshes near Great Yarmouth, while the police search for him in the town.
A German coaster has just tied up in the port, ready to unload, but there are problems on the ship, mechanical and animal.
Both the Captain's dogs, unknown to him, have been infected with rabies. As he goes to feed them there is an explosion and the little mongrel, Mukki, escapes while his attention is diverted.
Before long both boy and dog are on the same marsh. Add in a meeting with the marsh 'ghost' and the mixture begins to brew. It is soon a question of who will have the last laugh...