My Bride is a Mermaid OVA

A new semester begins where Nagasumi, San and the others are third grader seniors. Everyone got into the same class except Nagasumi got sent to the eighth middle school, class zero where in a classroom full of delinquents. Unable to cooperate with his new class, their homeroom teacher Shark Fujishiro plans to make a killing spree session on his first day of homeroom. Meanwhile, a new student has came to Sun and Lunar's class who turns out to be Lunar's Papa who is dressed in a female middle school attire along with an apron. This disgusts Lunar and makes a whole ruckus in the class. When Seto Gozaburo asks them to quiet down, Lunar's father tells him to not interfere and the two start a fight. Lunar and Sun leave to find Nagasumi and get him back to his seat to clear their situation.

The first half of the episode has Shiranui working in a maid cafe to pay off the damage she did to the store. Sun, Mawari, Satori, and Lunar offer to work there also, and a 'competition' begins between the four. At the end, the store is destroyed again. The second half of the episode starts with a flashback scene from when Sun was a child, playing with her friend, and they make a promise. Then, Lunar and Nagasumi are watching a horror movie in the house while the others are out. Lunar freaks out and a zombie girl appears in the house. A few things happen, and the so-called zombie girl tells Nagasumi that 'It's all your fault.' The next day at school, she appears again and attacks all of the students except Lunar, who cowers in the corner. Sun returns and finds that the zombie girl is her childhood friend, Saturn. Saturn leaves, and they make a promise once more.