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.