Growing up as rivals, Nao and Kyousuke constantly challenge each other in academics and sports, with Nao consistently emerging victorious. Their dynamic shifts when Nao falls ill at 17 and returns from the hospital transformed into a girl. Despite Kyousuke's unease around women, Nao embraces this change without hesitation. Set in a world where gender transformations balance the male-to-female ratio, 'Kanojo ni Naru Hi' follows Nao's journey as he navigates life in a new gender identity.