Living quietly in a mountain hermitage, Kasumi is drawn back into conflict when Phase-4 — one of her own clones — approaches her, followed immediately by an ambush from three others. She and Ryu Hayabusa fight them off and move to engage MIST directly.

MIST's plan centres on resurrecting Raidou. Scientist NiCO, recruited by Donovan to replace Lisa Hamilton, has developed a revival-of-life project requiring massive bioelectric energy. Honoka — revealed to be Raidou's biological daughter — is kidnapped and used as a power source. When that proves insufficient, Ayane is captured too.

Kasumi and Hayabusa intercept Christie and a prototype Raidou robot, receiving Ayane's distress signal mid-fight. At the MIST facility, Kasumi is forced to fight NiCO and a brainwashed Ayane while Hayate pursues a Phase-4 clone that has taken Honoka.

Once Honoka and Ayane are freed, the three siblings — Kasumi, Hayate, Ayane — pursue Raidou together. They kill him for the last time, leaving nothing behind.

For the first time in the series, Kasumi does not face her past alone.