There's a wonderful emerald necklace called The Green Marvel hidden somewhere in Haverford-Snuffley Hall. Sneaky Jasper Jellit takes a job there as a gardener, hoping to get his hands on the jewels. But the house is haunted, his two pet rats Rags and Bags give him no end of trouble, and then there's Mrs Knutmegg the cook, who sees immediately that Jasper's up to no good...