Valkyrie Drive: Mermaid

Following their battle, Mamori and Mirei are restrained by two other Liberators. While Mirei is locked away in a disciplinary room, Mamori wakes up in the care of Kasumi Shigure, commander of the Welter, who explains that all the girls on the island are carriers of something called the Armed Virus, who are divided into Exters, who can transform into weapons, and Liberators, those who can wield them. Charlotte Scherzen, one of the Liberators from before, then informs Mamori that she is to undergo a wedding ceremony to find a suitable Liberator as a partner. As Mamori is arranged to be partnered with a trainer named Tomomi Nukui, Meifon Sakura, a girl who enjoys running bets on Liberator fights, follows order to set Mirei free, allowing her to come to Mamoris rescue. Undergoing a Drive once again, Mamori and Mirei escape to the beach, where they properly introduce themselves to each other.

Surrounded by Welter soldiers, Rain kisses Mirei to transform into the Disarmment Buster Arm that forces Exters out of their weapon states, while Lady J uses her skills as a motorcycle to fight back Charlotte and Kasumi. Just then, Akira Hiiragi, Welters governor and allegedly the only male on Mermaid, arrives to prove Mireis innocence, having encountered Meifon earlier, and recalls the troops, much to Charlottes ire. After Lady Lady take their leave, Mamori starts taking an interest in Akira, much to Mireis dismay, while Akira tells Kasumi about his interest in Lady Lady, who function differently from the usual Liberator partnership. Later that night, Mamori follows a vision of Akira into the jungle, unaware that it is an illusion set by one of Charlottes followers to try and put her in Arm form to kidnap her. However, Mirei arrives in the nick of time, managing to overcome her opponents mental attacks and defeat her.

Meifon works with her partners in crime, Hyouko Yundou and Futaba Kirii, to steal more goods from Veste while also making money from gambling. Feeling she can make a killing from bets, Meifon arranges for Mirei to enter a beauty pageant being held at the Mermaid Festival. On the day of the festival, Charlotte gets chosen as the winner, with Meifon deducing that Hyouko and Futaba had switched out the ballots in order to kiss up to Charlotte. Discovering what had happened, Mirei and Mamori face off against Hyouko and Futabas decoys, while Meifon uses her love of money to transform herself into an Arm to fight against the real thing, coming out victorious. With Hyouka and Futabas crimes exposed, the pageant is revoked while Meifon delivers the prize to Mamori.

Lady Lady are asked by Akira to investigate a location said to contain the secret of Mermaid, bringing Mamori, Mirei, and Meifon along with them. Uncovering a hidden cave, the girls find an underground construction plant, where they are attacked by an Artificial Arm that Mirei finds all too familiar. Unable to drive with Mamori following the ordeal of her attack, Mirei unleashes the true ability hidden in her body to fight the Arm, but is still overpowered by it. However, Mamori, determined to protect Mirei, brings out enhanced abilities of her own and transforms into an enhanced arm, allowing Mirei to defeat the Arm. As the girls manage to escape the facility as it is destroyed, another island, having detected the Valkyrie Effect coming from the battle, sends a girl named Momoka Sagara to Mermaid to investigate.

Kasumi recalls how she and her partner Hibiki Kenjo, who has been in shock ever after being attacked on the island, was protected by Akira and made commander of Warter. With Akiras gender exposed, Charlotte, under Momokas influence, declares herself the new governor, sending soldiers to capture the residents of Atelier Torino, while Momoka and her allies use a mysterious power to inspect peoples bodies for something. As Mirei infiltrates Feste, having Mamori take Akira somewhere safe, she is attacked by Kasumi, who is forced to fight her upon learning that Hibiki is being held hostage by the Adel team. With Mirei having to use her Enhance ability to defend herself, Mamori manages to rescue Hibiki and reunite her with Kasumi. After Mamori joins Mirei to defeat the Adel using the Valkyrie Effect, they are attacked by Momoka.

Mirei struggles to fight against Momoka as she wields Mamori against her, but is aided by Kasumi, who is finally joined in battle by Hibiki, as well as Lady Lady and Meifeng. However, Momoka uses Mamoris Valkyrie ability to absorb other girls into her arm and become stronger, overwhelming the others in a single shot and absorbing them. Inside the blade, as the captives are attacked by tentacles that draw the Armed Virus out of them, providing Momoka with enough power to even destroy a satellite, Mirei manages to break free and reunite with Mamori, freeing her from Momokas control and releasing the others. Combining together into a perfect form, the Valkyrie Drive, Mirei and Mamori battle against Momoka as she combines with allies to transform into a giant robot. With support from all of her friends, Mirei manages to defeat Momoka, removing the Armed Virus from her and everyone else. As the residents of Mermaid are finally given the chance to return home to their families, Mirei and Mamori, along with Meifeng and Lady Lady, travel around the world in order to help others carrying with Armed Virus.