Following the success of their first two adventures, the intrepid pair join Betsy's mum, Myrtle the mermaid, beneath the waves.
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?