Ryunosuke loves baseball, thanks to the encouragement his mom gave him before she passed away. On the day Ryunosuke is set to turn in his permission slip for the third grade baseball team, his father tells him about his new adopted sister, Koharu Mori. Koharu is blind with prosthetic arms and legs.

Ryunosuke's father insists that since he can't be at home regularly, Ryunosuke must stop going to school in order to take care of Koharu. Ryunosuke hates giving up on his dream, resenting his new sister, but warms up to her after she saves him from a monster. When that monster reappears, his dad reveals the truth: his sister has an inhuman power that can destroy the world.

(Source: Anime News Network)

Note: Includes the one-shot "The Tales of Momo: Toshiharu and Friends".