Darcy Dolphin and friends are back for some more underwater adventures! In the third book of the series Darcy joins the Sea Trouts, whose motto is 'Lend a Fin'. But Darcy's attempts to be helpful don't exactly go swimmingly... and soon there's chaos at the pet exercise class, an extremely grumpy resident at the Retired Sharks home and a plankton on the run! Can Darcy sort it all out and still have time to throw Ozzie Octopus the best birthday party ever?