The Butler, Shadowing (その執事、尾行, Sono Shitsuji, Bikō) is Chapter 18 of the Kuroshitsuji manga.
Sebastian goes to wake Ciel up, who is already awake, much to the former's surprise. Ciel comments that it looks like Prince Soma and Agni stayed out all night, and he begins to read the newspaper, whose headline is that another mysterious attack occurred.
Meeting with Lau, Lau comments that Soma and Agni have been suspicious from the start, but Ciel says there is no reason for them to cause trouble. And if they were, they certainly would not come to live with Ciel. They decide to wait until nighttime to investigate Soma and Agni, who seem to leave every night. Sebastian, Lau, and Ciel follow them, and it turns out, Agni and Soma are simply looking for Mina.
They return to the manor and, shortly thereafter, Agni and Soma return as well. Close to two hours afterward, Agni leaves on his own, which Sebastian predicted. Lau, Sebastian, Ciel, and Soma decide to follow him, as they are all unaware what he does on his own at night. They follow him to Harold West Jeb's house, who is an importer of trade goods. They sneak onto his property where they are confronted by guard dogs. Sebastian appears to use some demon abilities to get them to back off. Lau calls them over, as he has already knocked out the guard personnel, much to Soma's and Ciel's surprise.
Inside, they overhear Harold West talking to Agni, who seems distressed. West states that, in one week, it will all be over. He has been planning on something for three years, and if Agni helps him, Mina would do something. Before he can say what, Soma bursts in, grabs Agni, and asks where Mina is. Ciel, Sebastian, and Lau continue to hide because West would recognize their faces.
Soma orders Agni to beat West, but Agni does not move. West then orders Agni to throw Soma out. In the hall, Lau and Ciel speculate this has to do with the black market and go to leave, but Sebastian says he will save Soma.
Soma questions Agni about what is going on and Agni tells him to leave. Distressed, Soma asks West what he has done to Agni. Upon West's orders, Agni moves in to hit Soma, but is stopped suddenly by a man with a deer's head. Outside, Lau is amused by Sebastian's use of a stuffed deer's head. West orders Agni to destroy the deer, but he says he cannot. Crying, he moves to do as West orders, but Sebastian's speed causes Agni to accidentally break many of West's prized possessions instead.
Lau, Soma, Ciel, and Sebastian flee West's manor and return to the Phantomhive mansion. They discuss Agni's amazing powers, and Sebastian says a human's blind faith can call out strength from within. Because of Agni's belief in his god, Prince Soma, he is able to do it as well. Sebastian says it is something a demon could never obtain, as it is bred from love and belief.
Soma destroys Ciel's tea set, wondering loudly why Agni betrayed him. He then flees to his room, and Sebastian says they should teach him some manners. He pulls Soma out of his hiding place, and Soma calls him a rude jerk. Sebastian says he is being rude for destroying other people's possessions as if it means nothing. Since they are not at Soma's manor, Sebastian does not have to follow his orders, calling him a spoiled brat. He goes on to say that Soma is powerless without Agni, who has betrayed him. Soma says he lost everyone, but Sebastian says they were never his to begin with, because his social status, servants, and wealth all come from his parents and he has earned nothing on his own. Soma says they were there for him, but Sebastian says it is only because it was their job to be, and they never really loved him.
Ciel enters the room and says he might have turned out like Soma, too, if "it wasn't for that month".