After an attempt on King Richard’s life, assassins target Princess Mary, the next in line to the throne. Team White Pigeon must protect her while locating deadly Cavorite bombs — a tough feat when Ange is Princess Mary’s sister in disguise.

While the funeral of the assassinated Prince Edward, who was the first in line to succeed the throne, is being held, the nobles of the Kingdom of Albion discuss topics such as whether the second-in-line Princess Mary or the third-in-line Duke of Arkham, Prince Richard, will succeed the throne, and how the Duke of Normandy will act.

This film consists of all-new stories that are direct continuations of the previous two films and are told in four episodic parts. The four short stories take place in March as Haruka Nanase is about to graduate from Iwatobi High School.

After falling for BC Freedom High’s clever deception, the Oarai Girl’s High tank-maidens find themselves faced with an entirely different battlefront than expected. However, Yukari suspects that any bluff that could have been carried out so easily by so many people must have some basis in fact. So, as the other school teams prepare for combat across the competition, Oarai digs down with a plan to undermine the groundwork beneath their opposition.

Too much tankery and not enough studying for her entrance exams has landed Momo in the hot seat! Luckily her friends won’t let her flunk out without a fight, and they’ve hatched a foxy plan to put her back on top. Miho’s stepping down to let Momo take up the mantle of commander, because if she can lead the team to victory in the Continuous Cup, Momo can retake her exams. The stakes have never been higher for the team as they fight for tank supremacy both on and off the combat field!

The story is set in London at the end of the 19th century. A new task has been assigned to Team White Pigeon, a group of spies from the Commonwealth. It is a delivery of a used bookstore owner to the Commonwealth. Ange, Dorothy and Chise successfully recapture the owner, who had been held in a detention center by the Kingdom. Meanwhile, the "spy hunt" became more intense in the Kingdom following an assassination attempt on the queen, and tension is strained at Control in the Commonwealth. Control speculates that the spy who has dispatched to the royal family might be a "double agent." To find out the truth, they order Team White Pigeon to contact with the spy, whose codename is "Bishop." How their new mission will be accomplished? And who is "Bishop"?