Valkyrie Drive: Mermaid

Synopsis

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.

(Source: Wikipedia)