Ellen and Robert Bunting from London have fallen on hard times, finding themselves on the verge of bankruptcy. But one night, a mysterious stranger, who introduces himself as Mr. Sleuth, takes the lodge with them for a rent they so desperately need to survive. The Buntings ignore the lodger's mysterious experiments and late night walks, relaxing into their good fortune. However, at the same time Mr. Sleuth arrives, a serial killer, who calls himself "The Avenger," starts terrorizing the city of London, leaving behind several dead bodies of young blonde women.
