Kagero: Deception II centers on Millennia, a human girl who has been brainwashed by the Timenoids — a race that was once human but long ago traded their souls for immortality. Humans perceive the Timenoids as demons and have invaded their territory, driving the Timenoids to desperate measures. They have placed Millennia in charge of defending their castle by trapping and eliminating the human soldiers who breach it.
But Millennia has yet to decide whose side she is truly on. The war between humans and Timenoids that shaped her situation is not one she chose, and the loyalties that were installed in her may not be the loyalties she keeps.