The first out of six movie sequels to the TV series.
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"?
(Source: Official Site)
— Description from AniList community (anilist.co).
