Francis, Alice and Laura are invited to spend a week on their uncle's game farm in the wilderness of eastern South Africa. Their holiday soon turns into a nightmare when they run into a gang of ruthless, heavily armed poachers. The mystery deepens: what is hidden in the tool shed next to the house? Why is the farm manager suddenly so evasive about it? Why does it look more and more as if their uncle is involved in organized crime? Cut off from the outside world, they have only their own wits to go on, as they try to solve the mystery of the Ten Horns.