Mai Kagari's decade-long love and obsession with her older brother... Though always seen as just friends, she remains in a relationship that's more than sisters should be. She has devoted her secret, twisted days to her reclusive brother, no one else knowing the truth. Her words, "It doesn't feel gross at all... Onii is my hero," echo like a declaration of love. Her tender care for her shut-in brother, now a NEET, symbolizes the depth of her affection. One morning, she accidentally discovers his morning wood, leaving him unable to refuse her advances. From that moment, her desires escalate uncontrollably, crossing the line between sibling boundaries. Their new relationship begins, using the messy room as their secret base, forging a deeper, forbidden bond. This secret will spiral into an endless love story that can never be broken.