Mabel lives with her parents in The Mermaid Hotel, by the sea. Mabel likes to keep an eye on the comings and goings of all the guests.

Then one day a particularly in-ter-est-ing old lady comes to stay. There is something very suspicious about her, with her growly voice and her heavy trunks and her beady-eyed tortoise. And why does no one know her REAL name?

There can only be one answer, Mabel decides … this guest is a SUPERVILLAIN. But even supervillains have a soft side, and as an unlikely friendship grows between the pair, their fantastical exploits take them well beyond the corridors of their seaside home.