Detective Murdoch investigates the murder of Edgar Leeman who was shot by someone when he answered a knock at the door. His last words, overheard by his wife were 'I'm sorry no'. There's soon a second victim Thelma Green and then a third, Robert Grimsby. They have little in common except to have perhaps come into some unexpected money. When constables Crabtree and Higgins find a wallet with $10 inside, they return it to address they find inside. They soon after learn that several such wallets have been returned to that address and Murdoch concludes that the killer is punishing those who don't. Murdoch devises a new camera that takes a photo every four seconds and sets it it on the street to see who might be leaving the wallets.