The movie is a continuation of the story from the original series that takes place half a year later. The original team joins an alliance to help take back the area around Unato Castle, however things do not go as planned. Will Ikoma and his friends be able to survive, or will they fall to the hordes of Kabane?

No longer truly human, Koyomi Araragi decides to retrieve Kiss-shot Acerola-orion Heart-under-blade's severed body parts that were stolen by three powerful vampire hunters. Awaiting him are Dramaturgie, a vampire hunter who is a vampire himself; Episode, a half-vampire with the ability to transform into mist; and Guillotinecutter, a human priest who is the most dangerous of them all. Unbeknownst to Araragi, each minute he spends trying to retrieve Kiss-shot's limbs makes him less of a human and more of a vampire. Will he be able to keep his wish of becoming human once again by the end of his battles? [Written by MAL Rewrite]

After helping revive the legendary vampire Kiss-shot Acerola-orion Heart-under-blade, Koyomi Araragi has become a vampire himself and her servant. Kiss-shot is certain she can turn him back into a human, but only once regaining her full power. Araragi has hunted down the three vampire hunters that defeated Kiss-shot and retrieved her limbs to return her to full strength. However, now that Araragi has almost accomplished what he’s been fighting for this whole time, he has to consider if this is what he really wants. Once he revives this powerful immortal vampire, there is no telling what she might do, and there would be no way of stopping her. But there is more to the story that Araragi doesn’t understand. If a newborn vampire like him could defeat the hunters, how did they overpower Kiss-shot? Can he trust her to turn him back to a human? And how is that even possible in the first place? Araragi is at his limit but he must come to a decision, and it may not be possible to resolve this situation without doing something he’ll regret… [Written by MAL Rewrite]

Kai, a young middle schooler, lives in Hinashi Town, a lonely fishing village, with his father and his grandfather, a sun-umbrella maker. He used to live in Tokyo, but after his parents divorced he moved back to his parent's home town. Kai has trouble telling his parents the complicated feelings he has for them, and he's lonely and pessimistic about his school life. One of his joys is uploading songs he writes to the internet.

One day, his classmates Kunio and Yuuho invite him to join their band, "SEIRÈN." As he reluctantly follows them to Merfolk Island, their practice spot, they meet Lu, the mermaid girl. Lu sings merrily and dances innocently. As Kai begins to spend time with her, he starts to be able to say what it is that he's really thinking.

But since ancient times, the people of Hinashi Town have thought that mermaids brought disaster. Something happens that puts a huge rift between Lu and the townspeople. And then, the town is in danger. Will Kai's cry for the heart be able to save the town?

(Source: Fuji Creative Corporation)