One day while Shirou was delivering a secret weapon to his colleague, he met Amane Mao, a super strong girl who is capable of lifting cars. Before they knew it, an ERROR, a monster that struck Old Tokyo 14 years ago, reappeared and started on a rampage. While he was arguing with his colleague over the use of the super weapon to save civilians, his colleague was blown away and died. Faced with death, Shirou had no choice but to combine with Mao with the super weapon and destroy the ERROR.

Mashiro-hime has just transferred to the all-girls school, Garderobe…but little do the other students know, she's actually a boy. Desperate to escape an all-boys school where he was looked down upon, he disguised himself as the late Mashiro-hime to get in. Unfortunately, due to "her" royal status, members of an organization known as Shwarz are trying to assassinate him, and all the while, he's trying to break his Contract with Arika Yumemiya, who unexpectedly became his Otome to protect him from a member of Shwarz.

Sequel to Mai-Otome. Set 3 months after the 1st series.