Following the success of their first two adventures Mr Tiger, Betsy and the Blue Moon and Mr Tiger, Betsy and the Sea Dragon, the intrepid pair join Betsy's mum, Myrtle the mermaid, beneath the waves, from bestselling author Sally Gardner and illustrated by Nick Maland.

Crumble cakes! When Betsy joins Myrtle the mermaid in her underwater world, she discovers there may be monsters . . . shipwrecks, lost treasure and secret cities. A seapig's prize seahorse, Pudding Pie, has been stolen by a cantankerous giant octopus and Betsy and Mr Tiger must go to the rescue in their new submarine. But what happens when a tiger's whiskers and tail are simply too long to squeeze into forgotten dark caverns where even the moon cannot shine, and Betsy has to take the final steps of the adventure on her own?

The paperback edition of this book is printed in blue ink in Dyslexie, a font specially designed to make reading easier - and more fun.