When the world-famous marine biologist, Dr Denman, comes to town there is much excitement. For budding scientist, Lewis, it's a dream-come-true as he might be able to work as her assistant. He might also be able to learn more about why his three friends, Cleo, Rikki and Emma, turn into mermaids everytime they're splashed with water. But as Lewis spends more time at Dr Denman's lab, Cleo and her friends begin to worry that they secret could be under threat. Could they end up as lab rats?