"Black Butler" is set in Victorian England and centers on Ciel Phantomhive, the young head of the Phantomhive noble family and the owner of the Funtom Corporation, a successful toy and candy company. After his parents are murdered and their estate is burned down, Ciel makes a contract with a demon named Sebastian Michaelis, who serves as his butler. In exchange for Sebastian's unwavering loyalty and assistance in achieving his goals, Ciel pledges his soul to the demon upon the completion of their agreement.

Ciel, now bearing the title of the Queen's Watchdog, is tasked with investigating and resolving criminal activities in the underworld on behalf of Queen Victoria. Despite his aristocratic demeanor and frail appearance, Ciel is a shrewd and calculating individual, driven by a desire for vengeance against those who wronged him. Sebastian, on the other hand, is a highly skilled and impeccably efficient demon, capable of superhuman feats. His unwavering devotion to Ciel is tempered by his anticipation of eventually claiming Ciel's soul.

The narrative unfolds through a series of episodic arcs, blending mystery, supernatural elements, and dark humor. Ciel and Sebastian navigate a world filled with aristocratic intrigue, occult phenomena, and grotesque adversaries. Their investigations often lead them into encounters with other supernatural beings, including reapers, angels, and rival demons, as well as human antagonists with their own nefarious agendas. The dynamic between Ciel and Sebastian is central to the story, with their master-servant relationship characterized by mutual dependence and underlying tension.

Supporting characters include the Phantomhive household staff, who, despite their comically inept appearances, each possess unique skills that occasionally prove useful. Notable figures include Finnian, the gardener with superhuman strength; Mey-Rin, the clumsy yet sharp-eyed maid; Baldroy, the pyromaniac cook; and Tanaka, the elderly steward who often appears more capable than he lets on. Other recurring characters include Elizabeth Midford, Ciel's cheerful and sword-wielding fiancée, and Undertaker, a mysterious and eccentric funeral director with ties to the supernatural.

As the story progresses, deeper layers of Ciel's past and the circumstances surrounding his contract with Sebastian are revealed. Themes of loyalty, morality, and the consequences of vengeance are explored, often through morally ambiguous situations. The series balances its darker elements with moments of levity, creating a tone that is both macabre and whimsical. The intricate plotlines and character dynamics make "Black Butler" a compelling exploration of power, servitude, and the human condition within a richly imagined historical and supernatural setting.