The first series of Black Butler followed Ciel Phantomhive, assisted by his demon butler Sebastian trying to bring to justice those responsible for the death of his parents, at which point Sebastian would consume his soul (as we reviewed here and here). It was a pretty complete series, where everything seemed well wrapped up, so how do they do a sequel series you ask? Not entirely well is the answer.
Hello. I am Sebastian Michealis, butler of Phantomhive Manor. Ciel Phantomhive is my young master, and I follow his orders to my death. We have made a contract: I serve him until he avenges his parents' deaths, and I get his soul. If you even attempt to injure him, or any member of the household, you will be dead. I'm simply one hell of a butler.