When the queen sends Ciel a letter notifying him about a string of kidnappings, she asks him to investigate. The first thing they notice is that every town that has been kidnapped from has been visited by the Noah's Arc Circus. So Sebastian and Ciel visit the circus, which seems just like any other circus at first glance. So rather than try and figure things out from outside they infiltrate the circus and become performers, to better understand what goes on behind the scenes.
Black Butler: Book of Circus was a better than the last season. I found out that Black Butler Season 2 can actually be skipped since it was never actually part manga, nor was anything past the cooking competition in season 1. The season remains mostly clean. There is a scene where a lot of the guys are shirtless as they pour water on themselves. There is also a scene where you see Sebastian put his arms around a girl to get information out of her. While you don't see anything inappropriate it's heavily implied that they were being far more intimate than one would usually be with an acquaintance. If you enjoyed the previous seasons I would give Book of Circus a try as well.