Kuroshitsuji Volume Six opens with Ciel and Sebastian beginning their investigation into a number of missing child reports. These disappearances seem to be connected to the travelling schedule of the Noah’s Ark Circus. (Their investigation begins at Scotland Yard. Abberline is extremely unhappy to have the pair going through the Yard records to get more information about the disappearances. He’s even less happy when Sebastian gives him money in exchange for going through the records.)
Now, the curry arc is about where the anime and the manga permanently part company. The closest we get to the circus arc in the anime is a bizarre situation involving a toymaker who turns girls into dolls. The toymaker in question seemed to have a strange obsession with Ciel. He was also connected in some way to the occultists who abused and attempted to sacrifice Ciel. (The occultists must have been extremely surprised when the demon decided to make a contract with the sacrifice instead of with the occultists doing the sacrificing.) I am mostly pointing this out because the ideas and themes in the Noah’s Ark Circus story and the toymaker story manage to be somewhat congruent despite the differences in tone and execution.
So, Ciel and Sebastian do some preliminary investigating by visiting the Undertaker, who after the usual fun and games gives them some interesting information. They learn that the missing children are possibly still alive because the Undertaker reports that none of the children had been his “clients.” (There is a slight logic break down here, since it is unlikely that “The Undertaker” is the only mortician in England. On the other hand there are definite hints that the Undertaker is some variety of supernatural entity, and in the anime it turns out that he is also a Grim Reaper.)
After an initial investigation where they discover that many of the circus members have very well-made artificial limbs, our protagonists decide to join the Noah’s Arc Circus under cover. They are not very good at it since they manage to stick out like sore thumbs. (The troupe members do not seem to think they are too strange however, possibly because carnies are mostly accustomed to weird.) Ciel and Sebastian soon discover they are not the only ones on the case. William T. Spears has also joined the circus, and he sticks out even more like a sore thumb than Ciel or Sebastian.
Since William is a Grim Reaper, he is less than pleased to find Sebastian at the Circus. He seems to feel that Sebastian is likely to be randomly after the souls Spears is supposed to collect. Sebastian learns from William that he has joined the circus because there will be a number of deaths in connection with the circus. He is extremely resentful of being on this job since he is covering for Grell. William’s presence makes Ciel and Sebastian’s investigation extremely difficult, since William will not let Sebastian do any investigating.
Despite the obstacle of William (and Ciel’s complete inability to dress himself or do simple chores) Ciel gets a chance to search the tents. He discovers that the circus has a benefactor who also supplied the disabled members of the circus with their prosthetic limbs. The volume ends with Ciel being discovered snooping by his tent mate.
The circus arc is a lot more interesting (and much more creepy) than the corresponding anime episodes. We get some hints into Ciel’s past, which both Sebastian and the Undertaker use against him. Sebastian uses Ciel’s past to bully him when Ciel wants to abandon the investigation. The Undertaker mentions Ciel’s past because the Undertaker is a creeper.