The Butler, Surmising is Chapter 141 of the Kuroshitsuji manga.
Undertaker states that he could not bear to lose another Phantomhive. Upon hearing his words, Sebastian says that he suspected something like that ever since they found out that one of Undertaker's mourning hair lockets was for Cloudia Phantomhive. However, Sebastian did not expect Undertaker's attachment to the Phantomhives to be so deep. Sebastian then asks Undertaker what is exactly the relationship between him and the Phantomhives, and Undertaker replies that he leaves it to his imagination.
Then, "Ciel" speaks up and says that this matter does not concern Sebastian and that servants should not speak before being allowed to—causing the servants to flinch. With a happier expression, "Ciel" turns his attention to Ciel and expresses his joy of finally being reunited with the "only family he has left."
Afterwards, Ciel proceeds to tell Undertaker that raising the dead is madness, but Undertaker's only response is to ask, who even cared if a person was alive or not as long as they were together, before starting to explain how he managed to bring back "Ciel"—his masterpiece who has no stitches, who grows and thinks and moves independently like a real human.
Sebastian asks whether the Bizarre Dolls they encountered on the Campania and at Weston College were byproducts of the experiments to bring back "Ciel," and Undertaker tells him that the Bizarre Dolls that they have seen until now were only the first mobile ones. The experiments itself started when he rescued "Ciel" in January 1886.
Surprised, Ciel asks where "Ciel" has been all the time then, and "Ciel" admits that he was always by his side: when Ciel needed information for the Jack the Ripper case, when he burned down Kelvin Manor, on the Campania, and during his time at Weston College.
While Ciel still stares at his brother in horror upon hearing his words, Grelle Sutcliff comes through a window and introduces themselves. Right afterwards, Othello walks through the door and announces that, as they have finally found their target, the "spanking" can now begin.