Set six months after the events of the "Blood-C" anime series, "Blood-C: The Last Dark" follows Saya Kisaragi, who is now a lone, vengeful figure in a dystopian Tokyo. The city is under the control of Fumito Nanahara, who has used his organization, Tower, and a puppet corporation, Seventh Heaven, to implement the "Youth Ordinance Bill," which enforces curfews and internet restrictions for minors.
The plot unfolds as Saya seeks revenge on Fumito for the elaborate experiment he conducted on her and the death of everyone she knew. Her quest leads her to an underground hacker group called SIRRUT, who are also working to expose Fumito and his human experimentation. Saya, with her immense power and skill in fighting the monsters known as Elder Bairns, becomes the group's "ultimate weapon." She works with them, though initially with a cold and detached demeanor, as their goals align.
A key relationship develops between Saya and Mana Hiiragi, a young member of SIRRUT who has her own reasons for fighting against Fumito. Mana's kindness and concern for Saya help to break down Saya's emotional barriers, which were erected after the trauma she experienced.
The film's climax involves a final assault on Fumito's Tower. Saya confronts Fumito, and a brutal battle ensues. Ultimately, Saya defeats Fumito and the Elder Bairns he has created, but not without significant personal cost. The ending leaves Saya truly alone, having achieved her revenge but without any clear purpose, as she abandons her sword and disappears into the shadows.