The protagonist, Osanai Haruki, is a overworked corporate employee who is recommended by his psychiatrist, Minato Sakura, to undergo a "regression therapy". He ends up revealing his repressed desire to regress into infancy and becomes dependent on Sakura and her younger sister, Yuki, who both have overpowering maternal instincts. The story deals with the consequences of Haruki's regression and how the characters interact and grow closer as a result.