Meri Pebble, professional spy mermaid, has a new mission. She’s going to be a bodyguard for the famous Sushi Sisters (known for a disastrous appearance on reality TV programme Catwalk Prawn, and for driving around in a submarine shaped like a giant bit of sushi). Beattie, Mimi, Zelda and Steve the talking seahorse are coming along for the ride as the least convincing glam squad ever. Meanwhile, on land, their human friend Paris is moving from California to a castle in Scotland complete with a loch rumoured to be haunted by a mermaid. The gang soon realise there’s something o-fish-ally fishy about the Sushi Sisters – but could it lead them all the way to Paris’s loch? And what has the loch got to do with things like mercats and hamstars (half hamster, half starfish)?