Nozumi and her brother where perfectly normal, with the usual ups and downs in an 'almost rivalry tone' relationship that every sibling has..... But suddenly, a cute and innocent tickling game goes out off hand, despise Nozumi's pleas to stop such event.

While trying to forget about it, Nozumi can't help but wondering: was such an horrible experience only a game to him? or there are any deeper feelings involved?... Either way, Nozumi just can't stop thinking about that issue, feeling more and more confused every time.