Each of the series revolves around girls who have been infected with a mysterious virus known as the A Virus (Armed Virus). These girls are divided into two classes; Extars, who can transform into weapons when sexually aroused, and Liberators, who have the power to wield an Extar’s weapon form, known as Liberator Arms, through a process known as Drive. These girls are brought to separate islands (which each of the series is named after) to spend their days completely isolated from the rest of the world until the islands’ Observers, authorized by the government Organisation AAA, ostensibly deem them ready to rejoin society again.
Mermaid follows an Extar named Mamori Tokonome who forms a partnership with the Liberator Mirei Shikishima. Bhikkhuni follows two sisters, Rinka and Ranka Kagurazaka, who are infected with a variation of the A Virus known as the V Virus.