September 19 marks the first day of Academy City's largest athletic meet, the Daihaseisai. Newcomers to Academy City, Kamijou Touya and Shiina arrive to look for their son Touma. By chance, they would soon meet a woman named Misaka Misuzu, Mikoto's mother. Shiina slowly develops a dark aura upon seeing her husband's friendly attitude towards Misuzu, only to calm down upon seeing Touma talking to what turns out to be Misaka Mikoto, the one Misuzu is looking for.
To their surprise, they are witnessing the start of an anything-goes punishment game between the two.