A peculiar transfer student named Ougi Oshino has just arrived at Naoetsu Private High School. She is quickly introduced to senior student Koyomi Araragi by their mutual friend Kanbaru Suruga, in hopes of obtaining advice regarding a strange discovery she has made. After taking a look at the school's layout, Ougi notices that a classroom has appeared in an otherwise empty area—a place that should not exist. Unsure if this is the work of an apparition, Araragi and Ougi attempt to unravel the truth behind this enigma. But Araragi soon discovers, after finding himself locked in with Ougi, that the room holds the memory of an event he had long since forgotten. [Written by MAL Rewrite]

Completing every job with ruthless accuracy, Train Heartnet is an infamous assassin with no regard for human life. Donning the moniker "Black Cat" in the underground world, the elite killer works for the powerful secret organization known only as Chronos. One gloomy night, the blasé gunman stumbles upon Saya Minatsuki, an enigmatic bounty hunter, and soon develops an odd friendship with her. Influenced by Saya's positive outlook on life, Train begins to rethink his life. Deciding to abandon his role as the Black Cat, he instead opts to head down a virtuous path as an honest bounty hunter. However, Chronos—and particularly Creed Diskenth, Train's possessive underling—is not impressed with Train's sudden change of heart and vows to resort to extreme measures in order to bring back the emissary of bad luck. This assassin turned "stray cat" can only wander so far before the deafening sound of gunfire rings out. [Written by MAL Rewrite]

Freshman Kaho Nikaidou is from an influential family, and as such, no one has ever said anything even remotely mean to her, for fear of incurring her household's wrath. Wishing to be around people who will not treat her as special because of her background, she leaves home to live at a dormitory for her new school year. Shortly after arriving, Kaho accidentally wanders into traffic and is saved by a tall, handsome stranger. When she asks for his name, he tells her she is weird and walks away. Having finally met the only person to ever say an unkind word to her, Kaho falls head over heels for her savior. After meeting her rescuer yet again and discovering that his name is Kanade Takahashi, she confesses her love to him. Kanade says he would like for them to be a couple, but that Kaho may not want to date him after she finds out his secret. To her shock, Kaho discovers the startling truth: Kanade is a fifth grader! Deciding that she can't date a fifth grader, Kaho intends to break up with him. But as she spends more time with Kanade, she begins to care for him even more and continues dating him. Hatsukoi Monster follows Kaho's first steps into love with Kanade, her immature, yet kind, fifth grade boyfriend.

Feeling slighted by Rin, Kuro goes on the hunt for a new master, but can he find anyone who can truly replace Rin?