Merman in My Tub summary: When an impish and self-absorbed water creature named Wakasa takes over the bathtub in Tatsumi's Tokyo apartment, the calm and collected teenager is faced with a challenge. Despite their vastly different personalities, Tatsumi and the mischievous merman form an unlikely bond. However, things become more complicated when Wakasa's aquatic companions, including a crab, jellyfish, and a sentient octopus, join the mix, creating chaos in Tatsumi's normally tranquil bathroom.