BLACK BUTLER: BOOK OF CIRCUS

In late-nineteenth-century England, the Noah's Ark Circus enlivens the city of London with their grandeur and spectacular stunts. However, children are mysteriously disappearing from the town in a manner correlating to the troupe's movement. The Queen, then, sends her notorious Watchdog, Ciel Phantomhive, on an investigative mission to retrieve the missing children. Ciel and his demon butler, Sebastian Michaelis, infiltrate the circus, disguised as team members, to study and possibly unearth its ulterior motive. This arc immediately follows the Curry Contest Arc in the manga, which ended at episode 15 in the first season of the anime. — Description from AniList community (anilist.co).