In 2010, an old man, Robert Fowlds, picks up a discarded note dropped by teenaged vampire Eleanor Webb, who has taken to writing her life story and then throwing the individual pages to the wind. Realising what she is, the old man invites Eleanor to his house and tells her that he is ready for death. Eleanor proceeds to kill him and consume his blood. Elsewhere, Eleanor’s mother, Clara, is chased from the lap-dancing club where she has been working by Werner, a member of the vampiric Brethren, who demands to know where Eleanor is. Clara decapitates Werner, burns his body, and leaves town with her daughter.
Eleanor and Clara seek sanctuary in the coastal town of Hastings. There, Clara sets her sights on a lonely soul named Noel, who has just inherited the Byzantium Hotel, a once-thriving business that has fallen into disrepair. Eleanor plays the piano in a restaurant and is approached by a young waiter named Frank, who takes a shine to her. Having seduced Noel, Clara turns the Byzantium into a makeshift brothel and Eleanor joins the local college, which Frank also attends. Interested in her past, Frank questions Eleanor, who writes her story for him to read. Not entirely believing it, he shows it to their teacher, Kevin.
The story, revealed in a series of flashbacks over the course of the film, begins during the Napoleonic Wars, when a young Clara encounters two Royal Navy officers, Captain Ruthven and Midshipman Darvell. While Darvell gives Clara a pearl and is kind to her, Ruthven convinces her to come away with him, to Darvell’s dismay. Ruthven, taking her to a brothel, rapes her and forces her into prostitution. Over the years, he visits her and is physically abusive. When Eleanor is born in 1804, Clara leaves her at the local private orphanage and secretly visits her at night. Years later, Clara is dying of what appears to be tuberculosis when the brothel is visited by Darvell, who has become a vampire. He gives Ruthven a map of an island where people can become vampires if they are willing to die. Clara shoots Ruthven in the leg, steals the map, and makes her way to the island to become a vampire. Darvell finds Clara and takes her to the Brethren, a secret society of vampires that protect the secret of vampirism.
As the members of the Brethren have traditionally been male nobles, they are appalled that a low-born female prostitute has joined their ranks, but they decide to spare her life, warning her that she must abide by their code, but that she may play no part in their society. Shortly afterwards, Clara’s decision to spare Ruthven comes back to haunt her when the vengeful syphilis-ridden captain turns up at Eleanor’s orphanage and takes her to the attic to rape her. Clara brutally murders him, but she is too late, and Eleanor is condemned to a slow and painful death. Desperate to save her daughter, Clara takes Eleanor to the island and has her transformed into a vampire, violating the Brethren’s code. The Brethren begin hunting Clara and Eleanor.
In the 21st century, Eleanor falls in love with Frank, who has had a life-long battle with leukaemia, and decides to help him turn into a vampire so that they can be together. Before killing Kevin, Clara finds out from him that Eleanor has told Frank about their past. Clara tries to prevent Eleanor from leaving the hotel and succeeds in trapping her in the lift. Noel is accidentally killed falling down the lift shaft. Darvell and Savella, the Brethren’s leader, pose as police and find out where Clara is, from Kevin’s colleague Morag, and go to kill her. Clara leaves without killing Frank when she realises that Eleanor is in mortal danger. The Brethren kidnap Eleanor and drive her to an abandoned fairground to destroy her, only to be stopped by Clara. After killing Morag, Savella battles Clara and ultimately subdues her. Savella hands Darvell his sword (which he took from Byzantium, during the Crusades), so that he can kill Clara. Darvell has always harboured feelings for Clara and kills Savella, instead. Clara gives Eleanor the money saved from running the brothel and tells her it is time for them to part ways. Clara and Darvell depart as companions, while Eleanor takes Frank to the island (filming location: Skellig Michael), where he becomes a vampire.