High school student Tatsumi lives alone. One day he spots a person while walking by a river. He approaches the figure and discovers a beautiful man collapsed on the ground.
Wakasa begins living in Tatsumi's bathtub. Just having a merman live in his home is difficult enough, but another major problem arises: the living expenses including water for Wakasa!
One weekend morning as Tatsumi heads to the bathroom with breakfast, he hears lively voices coming from inside.The puzzled Tatsumi opens the bathroom door and discovers a mysterious octopus? No, it's Wakasa's friend Takasu.
Tatsumi informs Wakasa and Takasu of a rumor about a ghost appearing in a nearby park. Behind Tatsumi, a mysterious white object appears... The white shadow tells a surprised Tatsumi, "Nice to meet you. My name is Mikuni."
A witch, a vampire, and a mummy... From the darkness, Wakasa, Takasu, and Mikuni appear.When Tatsumi sees Wakasa dressed as a witch, he wonders why. When Mikuni declares, "Trick or Treat," he realizes it's Halloween, but he tells the trio he has no candy. The watchful trio pounces on Tatsumi with a tickle attack! What will happen to Tatsumi? Will Wakasa and his friends acquire any candy?
Wakasa reads his magazines as usual. He had become obsessed with the female fantasy of taking a bubble bath. When Tatsumi enters the bathroom, drags Wakasa out of the tub. Wakasa doesn't understand.
The doorbell rings. A panicked Tatsumi says, "She's here! I'm counting on you," and Wakasa responds by giving him a thumbs up and saying, "Leave it to me!"
One day at dinner after Wakasa is finished eating, Tatsumi finds leftover bell peppers and carrots on his plate. When Tatsumi asks why he doesn't eat vegetables, Wakasa responds being a carnivore is the current trend.
Wakasa is shocked when Tatsumi leans over the tub and announces it stinks. Tatsumi cleans the tub responsible for the odor and transfers Wakasa to a children's pool. Saying, "Brother, tell me if you're using the pool," Kasumi mistakes Wakasa for Tatsumi and hugs him.
Some schoolboys are making a commotion in front of Tatsumi's apartment. They leave immediately when Tatsumi calls out to them. Looking down, Tatsumi discovers a conch rolling near his feet.
When Wakasa asks if something good happened, Tatsumi shyly admits his manager gave him a raise and allows Wakasa to splurge a little. Wakasa grins at Tatsumi's offer to splurge, and Tatsumi buys Wakasa a massive amount of milk. While Wakasa enjoys his milk bath, Mikuni, Maki, and Takasu arrive...
Wakasa gets nervous when Tatsumi is late coming home and gets out of the tub to look around the house, causing a mess and nearly drying out. When Tatsumi comes home, he thanks Wakasa for worrying about him, but makes him promise not to get out of the tub next time, or else he won't give Wakasa the cake he brought with him.
Wakasa worries that Tatsumi may kick him out after watching a man complaining about his partner's moocher tendencies on TV, and asks Tatsumi to help him find a job. Because Wakasa has nothing to put on a resume, Tatsumi recommends that he enter by-mail sweepstakes, but Wakasa is unable to write kanji and Tatsumi must teach him. Wakasa fills out stacks of postcards, but is shocked that he will not get to see the contest results for a month. Tatsumi reassures Wakasa that he doesn't have to worry about being kicked out, because they are friends. Wakasa mainly enters contests to win free meat, but what he receives at the end of the month is a scarf, which he had been aiming for to give to Tatsumi. During the credits, Wakasa's friends come over for a New Year's party, and Kasumi also has a brief appearance wishing her brother a happy new year from her own house.