Valkyrie Drive: Mermaid

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.

Stressed from work, Akira, who is revealed to be a woman, sneaks out to be spend some time with Torino, keeping her identity a secret from Mamori and Mirei. Spending the night in Mamori's room, she recalls how her mother arranged for her to act like a man upon arriving at Mermaid in order to stay safe. The next day, some rebellious girls take one of the Ritters hostage in the building where Akira is keeping her male clothes. As Charlotte and Kasumi argue over how to approach the situation, Mamori, Mirei, and Akira follow a secret passage into the building in order to rescue the hostage, only to fall into a trap laid out by the Ritter, who was sick of her current life. With Mirei knocked out, Mamori has Akira drive with her in order to stop the terrorists, briefly exposing her identity to Mirei before leaving the scene.

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.

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.

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.

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.

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 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.