The world famous Karthoum diamond, which is worth some 2 million extremely hard Dutch Guilders, is being transported to the Netherlands from Pretoria to be on display in a museum. Director Sondag feels the diamond isn't getting enough protection and resorts to hiring a private detective to keep an eye on it. Unfortunately for him, some Italian mobsters are planning a jewel heist and make sure Sondag hires the bumbling and incompetent Sherlock Jones. In fact, Sherlock's basset hound Watson has a better grasp of the situation than his master.