On a summer afternoon under the blazing sun, Aki, a boy who’s often bullied, decides to confess his feelings to his childhood friend, Yui. However, on their way home, Yui suddenly disappears. Even though they had been walking together, she vanishes without a trace. Losing Yui — his only ally and the girl he loved — Aki falls into despair. But then, Yui appears before him once more. "I’ve become a shadow." Her appearance was eerie. She extended her arms and legs from within the shadows, moving as if swimming through them. As long as there was a shadow, she could come and go freely — through the ground, the ceiling… even through people’s bodies. Was she really Yui... or something else entirely? Desperate to bring Yui back, Aki sets out in pursuit, but against his wishes, the nightmare of the “shadow” begins to consume them both.