Enormouse is bigger than all the other mice in the mousehole - and he looks different too. He is teased by the other mice, even though he takes care of them, and when the mice find a picture of a rat they all decide that Enormouse is actually a rat. Poor Enormouse leaves the mousehole and goes to join the rats. But what he discovers is that looks aren' t everything. The rats are dirty and untidy, and he is not at all like them. Now he realises where his true home lies... Meanwhile the mice have also realised their mistake. They miss Enormouse and set out on a dark and dangerous journey to find their friend and bring him home.