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.

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"?

The fourth film of Princess Principal: Crown Handler.