When Kurumi Hanazono encounters her childhood friend Nagio at her school, she almost fails to recognize him. Overcome with a rush of nostalgia, she visits their childhood secret place but is embarrassed when she recalls a vivid memory: her and Nagio sharing an innocent kiss before he left town.

Nagio is back now, and he surprises Kurumi by showing up at their secret location. As they reminisce about the past, Nagio does something unexpected, deftly planting a kiss on her lips. Shocked, Kurumi realizes that the current Nagio not only looks different―he is also mischievous and unpredictable! However, Kurumi's spartan brother Ataka cannot stand any boy who tries to get cozy with his precious little sister. When he gets a hint about what is going on between the two, he throws a massive fit and vows to drive Nagio away from his sister's life.

Wanting to pursue the relationship despite the risks, it is up to Nagio to find ways to steal kisses from Kurumi while her brother looms over him menacingly.