His rule: No sexual relations until age 18! Her rule: No rules. 
Free-spirited Yuri Hasegawa and straitlaced Keiichiro Katsuragi have fallen in love. But his elite political family--producing a line of prime ministers--does not allow male descendants to engage in any sexual relations until they are 18. Can the physically affectionate Yuri and rule-abiding Keiichiro keep their relationship strictly chaste? 
The couple has finally headed off on their romantic school trip. But Yuri gets ahead of herself and gives Keiichiro a kiss on the forehead, which suddenly triggers the Katsuragis into high-alert mode! Keiichiro's older brother, Shotaro, arrives on the scene. At first the two think he's come to give them a scolding, but something is off about the way he's acting!