Valkyrie Drive: Mermaid

Mamori Tokonome, a girl who was diagnosed with an irregularity in her body, is captured by agents and taken to a remote island known as Mermaid. There, she is attacked by two girls named Miyasato and Kouzuki, the former being a Liberator who arouses the latter, an Exter, to transform her into a gun known as a Liberator Arm. Just then, another girl by the name of Mirei Shikishima arrives on the island, protecting Mamori by transforming her into a sword, allowing her to defeat Miyasato and Kouzuki and escape to the jungle. Arriving at a white castle, Mamori and Mirei are trapped in a colloseum, they are confronted by another Liberator, Saejima, who uses one of her masochistist slave girls to bring out a ball-and-chain Liberator Arm. Just as the two are cornered, Mirei once again uses arousal to transform Mamori into the sword Liberator Arm, allowing her to defeat Saejima in an instant.

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 Mamori's rescue. Undergoing a Drive once again, Mamori and Mirei escape to the beach, where they properly introduce themselves to each other.

Mamori and Mirei are picked up by Meifon, who takes them to a small town run by Torino Kazumi, who takes care of girls who don't want to or are unable to stay at Veste. Meanwhile, despite Kasumi's attempts to keep it a secret, Charlotte learns about Meifon repeatedly stealing supplies from Welter, commanding Kasumi to go and apprehend her. The next day, as Kasumi arrives in Torino Town to search for Meifon, Mirei offers to take responsibility for the theft and turn herself in. When Charlotte prepares to attack her, however, Mamori comes to Mirei's aid, helping her to fight back. Just as they are overwhelmed by Kasumi, who doesn't use an Arm to fight, they are aided by two women, Rain Hasumi and Lady J, who together form the duo of Lady Lady.

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, Welter's governor and allegedly the only male on Mermaid, arrives to prove Mirei's innocence, having encountered Meifon earlier, and recalls the troops, much to Charlotte's ire. After Lady Lady take their leave, Mamori starts taking an interest in Akira, much to Mirei's 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 Charlotte's followers to try and put her in Arm form to kidnap her. However, Mirei arrives in the nick of time, managing to overcome her opponent's mental attacks and defeat her.

Mirei, Mamori, and Kasumi come across a girl named Nimi Minimi who, after running away from the castle when Charlotte tried to force her and her girlfriend Noe Oya into separate partnerships, somehow wound up in a giant form. Nimi is brought to Atelier Torino, where she manages to get along with everyone despite her gigantic form. The next day, as Nimi suddenly grows even larger, Mamori encourages her to go and see Noe and tell her how she feels. This naturally causes panic as she approaches the castle, with Kasumi's attempts to stop her only causing her to grow larger still. Before Charlotte can attack, Mirei and Mamori break Noe out of detention so that she can reach Nimi, allowing her to return to normal size. Akira then explains that the growth was a cause of Nimi's powers, which had simply taken a while to activate, allowing Nimi and Noe to remain partners with each other.

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 Futaba's 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 Futaba's crimes exposed, the pageant is revoked while Meifon delivers the prize to Mamori.