Fifteen years ago, disaster struck human civilization, and now dangerous man-eating monsters roam the ravaged lands, posing an existential threat to the remaining survivors. Amid this turmoil, an isolated facility shelters children and nurtures them in peace. However, as a few among them find out about the world beyond the narrow periphery of their nursery's walls, their curiosity about it slowly grows. Meanwhile, in the outside world, young survivors Maru and Kiruko band together to search for a special place called Heaven, each for their own reasons. Carrying past burdens and tragic secrets, the two hope to find answers to the cruelty they have experienced in their lives and in the world, which still remains in tatters.

During recess, Olivia, a foreign transfer student who doesn't know English, plays a game of look-the-other-way with Hanako Honda, a loud-mouthed airhead. Their rowdy behavior spurs the ire of Kasumi Nomura, a deadpan loner constantly teased by her older sister for her tendency to lose games. Not willing to compete, Kasumi declines Olivia's offer to join the fun, but eventually gets involved anyway and dispenses her own brand of mischief. Soon, a strange friendship blossoms between the peculiar trio, and they decide to form the Pastime Club, where they are free to resume their daily hijinks. Whether it be failing to learn English, trying desperately to become popular, or getting caught by teachers at the wrong time, school life will never be boring when the girls of Asobi Asobase are up to their hilarious antics.

Shuuichi Kagaya is what one would consider an average high school student, but sometimes, he turns into a monster. He doesn't know how or why he got his abilities, only that he would prefer no one knows about them. One night, he finds a building ablaze with a girl trapped inside. Deciding to save her, he transforms and carries her to safety, but accidentally drops his phone. The next day, the girl he saved—Claire Aoki—finds him and confronts him about his monster identity. She even goes so far as to push him off the school roof to prove her theory after Shuuichi denies her allegations. Desperate to save himself, he transforms, and Claire snaps a picture in order to blackmail him into telling her everything he knows about monsters, which, ironically, isn't much. As it turns out, Claire has a secret of her own: she has been searching for her sister, who also became a monster. She enlists Shuuichi's help to track her down, but they aren't the only ones searching for answers.

Miko Yotsuya's eyes water as she fixates on a single spot on her phone—she ignores yet another dreadful, horrific monster that is in her face, uttering the disturbing words: Can you see me? Before now, Miko enjoyed her unassuming high school days, with late-night horror shows serving only as a form of entertainment. But ever since one fateful day, she is the only person aware of the invisible monsters walking freely among humans. Courageously, Miko makes a bold decision: she will never, under any condition, acknowledge the presence of the horrid specters. However, even though she pretends they do not exist, she can still see how they disturb the people around her, especially her best friend, the energetic and lovely Hana Yurikawa. In order to protect them from the monsters' annoyances, Miko gives it her best to continue her school life and avoid every troublesome crisis—even when they scare her to tears.