Six months ago, Lord Hugh Anthony Disward, also known as Huey, lost his eccentric grandfather, Sir Wesley Disward, who was a renowned collector of rare books. His grandfather's will states that, in order to inherit his manor and everything inside it, he must take guardianship over the Bibliotheca Mystica de Dantalian—an archive that contains forbidden knowledge—and also take care of a mysterious girl called Dalian. As Huey settles into the manor, an old rival of his grandfather's arranges a meeting with him. Dalian, knowing the rival to be Wesley's killer, tags along and discovers that the murderer is in possession of a Phantom Book—a cursed tome that Wesley tried to seal away. When the book puts the two in danger, Huey discovers that the Bibliotheca Mystica de Dantalian and Dalian are one and the same, and she entrusts Huey with the key to unlocking the knowledge stored within her. Together, Dalian and Huey seal the book away, and thus begins an unlikely partnership as they solve mysteries caused by other Phantom Books.

Hayate Ayasaki's misfortune continues to hand him the short end of the stick. Now settled into his routine at the Sanzenins' mansion and Hakuou Academy, the butler continues to work as hard as ever in caring for his young mistress Nagi while studying the school's grueling curriculum—all on top of trying his best to survive the multitude of troubles that life relentlessly pushes onto him. The unintentional chick magnet's life is far from quiet, forced to deal not only with Nagi's yet unnoticed infatuation with him, but also the evergrowing string of accidentally seduced girls—most notably, the exceptionally plain Ayumu Nishizawa, a friend of his before he became a butler, and the diligent Hinagiku Katsura, Hakuou's student council president. While one struggles to gain his affection and the other desperately tries to deny her own, both the girls are going to make the boy's life a little more complicated than it already is.

On a train to her new middle school, Momoka Sonokawa is preyed upon by a molester, only to be rescued by a girl wielding dual pistols. The girl turns out to be Miou Ootori, Momoka's upperclassman and president of the Aogiri Academy's Survival Game Club, called Sabage-bu for short. Noticing Momoka's aptitude with a pistol, Miou takes a liking to her and tries to persuade her to join the club. In spite of her steadfast refusal, Momoka inexplicably finds herself a member of Sabage-bu. Sabage-bu is anything but a typical middle-school club. Its members' socially awkward behaviors scare Momoka. Their personalities range from the fanatical to the completely delusional. They do not hold back from walking around public areas wearing ghillie suits or even bungee jumping from the school building. Thrown together with this collection of survival game maniacs, the zealously ordinary Momoka must somehow adjust to her unexpectedly wild school life.

Naïve 16-year-old Mamori Tokonome is accustomed to being teased at school for having an unfortunate surname that can also be read as virgin. However, young Mamori will soon have to get used to being teased in other ways... Kidnapped during gym class, Mamori wakes up only to find herself stranded and under attack on the exotic island of Mermaid. Luckily, enigmatic fellow castaway Mirei Shikishima knows exactly how to take the lead—through a passionate kiss, Mirei unleashes Mamori's Exter transformation abilities, turning the innocent red-head into a battle-ready cutlass through the power of arousal. The duo will need to tap into that power as Mermaid Island is full of potential friends and foes: Charlotte, the sadistic Liberator of an Exter harem; the gluttonous and crafty Meifon; the mysterious but charismatic Akira Hiiragi; and the erotic biker duo Lady Lady. Mamori and Mirei's powerful and intimate embrace is the only way for the pair to ensure their survival on this scandalous island.