After losing their parents in a terrorist attack, the younger sister An is unable to heal from her emotional trauma and withdraws into isolation. Carrying her along, older brother Tart begins a desperate journey, fleeing in despair. As they evade assassins hunting them down, the two are forced to spend their days in tight, confined spaces. Amidst this closeness, Tart can no longer suppress his deep attachment to his sister, and one day, he finally loses control, unleashing his emotions in front of An. But in that instant, something shifts within her heart. "O... Onii-chan!!"