Koyomi Araragi has recently survived a vampire attack and met several girls plagued by supernatural entities. On top of all this, he wakes up one morning to find himself kidnapped and tied up by his girlfriend Hitagi Senjougahara. Araragi's sister Karen has run afoul of Deishuu Kaiki, a swindler who once conned Senjougahara's family. As a result, Senjougahara has imprisoned Araragi to keep him safe from the devious Kaiki. But when Araragi receives a frantic message from Karen, he sets out to rescue her.

Along with Karen's troubles, Tsukihi, Araragi's other sister, faces issues of her own. And when two mysterious women step into their lives, not even Araragi can anticipate their true goals, nor the catastrophic truths about his family soon to be revealed.