Yusuke Sawada had just been transferred by his parents into a new school, Aiho Academy. But, unfortunately, his parents forgot to enroll him in the dorm, and the apartment he was planning on staying in burnt down as he arrived. While complaining about his current situation, he is mistaken for a panty thief and attacked by a girl, Asumi Hirota, who happens to be his former childhood acquaintance. After realizing who he is, Asumi assumes that he is there to threaten her, and becomes uneasy. Asumi, believing Yusuke has something on her, decides to do something for him under one condition: he must not tell anyone her secret. However, Yusuke does not know what the secret is, but decides to pretend he does. After Asumi explains her condition, Yusuke asks if Asumi can give him a place to stay, Asumi agrees and takes him to the girl's dorm, but, because the dorm doesn't let males in the dorm, she must sneak him in. After meeting the roommates, he is accepted (with some debates) into their room.
Source: Wikipedia, "[en.wikipedia.org/wiki/Heart_de_Roommate Heart de Roommate]", available under the CC-BY-SA License.