Mermouse mystery by Philip Reeve

Pedro is getting used to the thrilling life of the Adventuremice. But one stormy night, he finds something he’s only ever dreamed of seeing: a real, live mermouse! This little mermouse is lost, and Pedro and the Adventuremice need to get him safely home. They must summon all their bravery and brilliance, plunge into the depths of the ocean and find the mysterious Mermouse City. It won’t be straightforward, but it will be another exciting, friend-filled adventure for Pedro and the team!