Gran has been particularly naughty lately and is sentenced to a stay in an old people's home, but the home is the stuff of nightmares for Gran, and it's soon up to the Spuds to break her free.

Dave stumbles across a mermaid at the dockside who invites herself round for tea and takes up residence in the Spuds' bathtub, so Dave and his family must somehow persuade their unwanted guest to go back to her watery home.