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? 
Yuri's attempts at having a normal romance with Keiichiro are thwarted on a daily basis. But now that they're on their first overnight school trip together in the mountains, they end up in the same bed?!