During the university entrance ceremony, Kyouya and Yuuki share a fateful encounter. Kyouya nicknames Yuuki "Tenshi-chan" and starts expressing his love in slightly twisted ways- following the latter around (and making sure not to get noticed) as well as collecting things like erasers and straws he's used.
One day, Kyouya picks up Yuuki's keys, sneaks into his house and then plants a listening device. From "good mornings" to "good nights", Kyouya was filled with joy listening to the sounds of Yuuki's everyday life at home.
The seasons change and the next summer arrives. Kyouya's hard work was paying off, and the distance between him and Yuuki shrinks; going from "we're in the same class and know of each other" to "friends in the same group". "At this rate, as long as we grow closer, we'll become lovers soon." The moment Kyouya thought this, shadows of another guys started to loom over Yuuki. Anxious about Yuuki's purity, Kyouya steels himself, planning to aim for when Yuuki's away to sneak into his house and uncover the identity of the mysterious third guy.
So the fateful day to carry out his plan arrives... After confirming that Yuuki's definitely away from home, Kyouya opens the forbidden door (using a copied key he made without Yuuki's permission). He goes through Yuuki's trash, lies around on his bed, fantasizes inside his bathroom, borrows his stuff... Kyouya does whatever he wants as he works hard on investigating.
But then, Kyouya starts to see an unlikely side of Yuuki he never knew...