From MangaHelpers:
Since their mother's death, sisters Tomoe and Mikage have been mostly raising each other. The older sister, Tomoe, is determined to get good grades and work her way into a good college. However, she's disturbed by the trouble Mikage sometimes gives her. For instance, at the beginning of the manga, Mikage sneakily locks a collar around her older sister's neck, forcing her to go through the day wearing it. Mikage wants Tomoe to think only of her, and her affection may be beyond sisterly love. However, at school, Mikage has made a new female 'friend' that has a serious crush on her. The two sisters have their separate lives at their respective schools, but their life together and their relationship may be most important of all.

Kouichi has looked forward to becoming a high schooler for one reason: to help continue his grandfather's coffee shop. But on the first day of school, a strange girl wearing an eerie mask tells him that she has come to take his life. She tells him that the heart beating within his chest is her own, and that she is an alien invader who has come to take over the world.