After catching a cold by giving his only umbrella to a girl sitting in the rain, Amane Fujimiya only expects her to eventually return it. However, Mahiru Shiina, the Angel of Amanes school and his neighbor, instead gives him a lot more than that. Instead of just the umbrella, she insists on returning the favor by helping him recover from his cold. However, she is oblivious to the battlefield that is Amanes apartment.