Mikoto has contracted an incurable illness that causes her to have seizures if she doesn't suck a penis for over an hour. The cause of this disease remains unknown, no cure has been found, and spontaneous recovery is her only hope. For Mikoto, her only salvation is her older brother. As long as he's around, she can immediately perform oral sex when a seizure strikes, allowing her to go about her day with peace of mind. Her brother's presence is what supports Mikoto's daily life.