Tumtum and Nutmeg are leaving the confines of Nutmouse Hall and voyaging on their most challenging adventure yet... at the seaside! Arthur and Lucy are going to stay with their Uncle, and Nutmeg is determined to keep an eye on the children while they are away. But of course they won't be alone as General Marchmouse decides to join the fun. And soon The General gets his wish as the mice find that their new friend, Lord Seamouse, needs their help. Together they must set out on an adventure to regain his lost treasure and stop his enemy, Purple Claw, from getting to it first.