Plot
Blood-C: The Last Dark, which follows the events of Blood-C, is set in a version of Earth where humans are preyed upon by monsters called Elder Bairns; to keep humans safe, the Elder Bairns honor a covenant dubbed Shrovetide, a contract between humans and Elder Bairns that allows Elder Bairns to feed on humans for a limited period of time. During the events of Blood-C, Saya fights Elder Bairns attacking her village. It is eventually revealed that Saya is an Elder Bairn barred from drinking human blood by an oath—captured by Fumito Nanahara, head of the organization that supervises Shrovetide, she entered a wager in which she would subject herself to an experiment, with her prize being the reneging of her oath. The experiment fails, and everyone involved is massacred aside from Fumito, his ally Yūka Amino, and Saya herself. Betrayed by Fumito, Saya swears vengeance against him. By the events of The Last Dark, Fumito—operating through his covert group Tower and its puppet corporation Seventh Heaven—has gained control of Tokyo's government and implemented the Youth Ordinance Bill, a law which enforces curfews for minors and regulates the use of the internet.
Six months later in Tokyo, Saya saves a girl named Mana Hīragi from a berserk humanoid Elder Bairn. Mana and Saya are rescued by Mana's comrades in SIRRUT, a covert group resisting Fumito's group who heard about Saya's past through Itsuki Tomofusa, a spy who died following the experiment. The group is led by Fumito's relative Kuroto Mogari, whose family was killed so Fumito could have control over the Shrovetide. Kuroto offers Saya SIRRUT's aid finding Fumito in exchange for her help. Hunted by Tower, and after saving Mana from Youth Ordinance Bill enforcers, Saya agrees to help. Saya and Mana infiltrate a private academy funded by Fumito when he is scheduled to give a lecture. When Saya attacks, it is revealed to be a trap baited with a false version of Fumito, and she must fight off an Elder Bairn before they can escape. In the aftermath, Mana reveals that her father disappeared after she helped him with his research into Tower. Regaining her confidence as a hacker, Mana locates Tower's main base, allowing Saya to infiltrate the base.
Facing the task force unit, she fights the head of Tower's security forces Kutō, killing him after he uses a vial of Saya's blood to transform into an Elder Bairn hybrid. Finding Fumito in the building's basement, she also finds Kuroto with him, before succumbing to a drug given by Kuroto. Fumito and Kuroto—whose family has the power to control Elder Bairns—were working together, with each killing the others' families to take control of Tower and Seventh Heaven. Saya also learns that the Elder Bairn who tried to kill Mana was Mana's father. When Kuroto attempts to kill him to control Shrovetide, Fumito stabs him with a phial of Saya's blood, transforming Kuroto into an Elder Bairn seed and killing him.
Fumito reveals his intent to dissolve Shrovetide using Saya's blood as the ratio of humans to Elder Bairns has shifted drastically due to the Elder Bairn seemingly dying out. As Saya recovers from the drug, Fumito fuses with the seed, which hatches into a giant Elder Bairn which Saya slays. A weakened Fumito reveals that he transformed himself into an Elder Bairn hybrid to become closer to Saya and free her from her oath not to feed on humans. Accepting that he has lost their wager, he impales himself on Saya's sword. Saddened at being left alone again, Saya does not return to SIRRUT, instead opting to disappear. In the aftermath, Yūka becomes Tokyo's governor and repeals the Youth Ordinance Bill, the members of SIRRUT return to a normal life, and Mana begins searching for Saya as the glimpse of her appears on the street of Tokyo.