Tiny Haruhiko.Mai is seen protecting a young boy against a large Sand Phantom. Several days earlier everybody had been wondering what they were like as children. That night Haruhiko finds a box of items from when he was a child. The next morning Haruhiko wakes up a child version of himself with no memory of being a teenager. He heads to his old junior school where he believes he is in first grade. He is found by Kurumi who returns him to the club. It is decided that he should still attend his high school classes while Mai ends up volunteering to look after him at her house. Although polite and intelligent for his age Mai still struggles to attend school, work, fight Phantoms and care for Haruhiko and it takes a toll on her health. Haruhiko realizes Mai is struggling and does his best to be as little trouble as possible, so Mai takes him to the park. At the park Mai appears close to exhaustion and when an angry Sandbox Phantom, tired of being used by cats as a toilet, begins attacking Mai ends up collapsing. Unable to protect Mai, Haruhiko makes a wish to stop being a child and suddenly reverts to his grown up self again. He quickly summons the demon Cthulhu and easily defeats the Phantom, saving Mai who is shocked as Haruhiko is in only his underwear, having outgrown his clothes. Back at his home, Haruhiko, who does not remember becoming a child, wonders if it happened because he read a letter he wrote as a child describing a happy Sunday he spent with his parents. Mai knows the letter is a lie, Haruhiko's childhood fantasy, and wonders if Haruhiko's powers made him a child to experience that fun for real, in the same way Kurumi's powers transported her to her fantasy world to overcome her fear. Haruhiko is teased by the girls about how cute he was as a child.