Shuntarou is a perverted older brother loathed by his sisters, Ayaka and Fuyuno. After playing a mysterious adult game, he wakes up transformed into a young boy and pretends to be an online friend named Koharu to avoid their abuse. To his surprise, his sisters are instantly attracted to his new form, leading to a series of erotic encounters as he enjoys the affection he always craved while hiding his true identity.
Shuntarou finds himself the center of a heated competition between his sisters, Ayaka and Fuyuno, who vie to provide the most "care" for his cute alter-ego, Koha-kun. After discovering a surprisingly gentle side to his sister Chiaki and returning to his original form, Shuntarou is shocked to be transformed back into a boy once again. He soon learns that Chiaki has developed a mysterious smartphone app that allows her to trigger his transformation at will, ensuring she gets her own turn to dote on him.