Koichi takes care of his sister, who has recently returned from a trip abroad in the United States as she is not well. While caring for her, he records evidence of ghosts in their home.
Akira is a normal high school boy, but he nevertheless impulsively licks some drool left by the mysterious transfer student Mikoto. The next day he falls asleep with an inexplicable fever. After five days Mikoto suddenly comes to visit, and makes him consume her drool. The fever instantly vanishes, and Mikoto explains that he's suffering from "love sickness." She can transmit emotions through her drool, and the two of them are now bound together.
After trying to rescue a man on the subway tracks, two teens wake up in a room dominated by a mysterious black sphere that sends them to hunt down and kill aliens hiding on Earth.
