On their way home from school, high school student Jin Okuhara is about to confess to his childhood friend Komachi Amamiya when the two are abruptly transported to another world. They fall into a dark, deep hole known as the Billbaim Abyss, a forbidden land described as being filled with madness and lewdness. In this hostile environment, they are immediately attacked by a giant tentacle creature. In an effort to protect Komachi, Jin is devoured by the monster and seemingly loses his life.

However, Jin does not die. He regains consciousness to discover he has been reincarnated as a small, weak tentacle monster—a fragment that was spat out by the very creature that consumed him. Struggling with his new monstrous form, Jin realizes that his tentacle body comes with powerful instincts, including an aggressive urge to attack females. He fears that his consciousness will eventually cease to be human as these urges grow stronger. After this traumatic transformation, Jin is rescued by a group of explorers who have also taken in Komachi. Together, they form a survey squadron dedicated to investigating the mysteries of the abyss, which they dub the Tentacle Hole.

The series follows Jin as he grapples with his dual existence. Initially, his tentacle form is small, measuring only about 63.5 cm, but he possesses the unique ability to transform into a humanoid shape by accepting the full power of his origin: the Vismaria Class. As a fragment of this strongest type of tentacle monster, Jin is still connected to a hive mind that bombards him with aggressive and sexual desires. He learns to use his abilities for combat, manipulating his appendages to form weapons or giant hands, utilizing raw strength, high-speed regeneration, and producing a healing mucus. The squad soon discovers that tentacle creatures have a specific weakness: a crystal core that, if destroyed, prevents their regeneration. They are also easily distracted by the scent of women, a flaw that survey teams exploit to leave their cores open for attack.

The narrative introduces a variety of characters who inhabit this dark world. Komachi remains Jin's primary emotional anchor and love interest, his will to protect her serving as the key to regaining control over his monstrous side. The survey squadron includes hardened members like Captain Dazune, Captain Aishu, and fighters such as Hiori and Lingue, some of whom have traumatic histories with the tentacles. Not all humans are allies; some squads use girls like Lylara as tentacle bait, abusing them for financial gain. As the story progresses, the squad faces a new, intelligent threat known as Liao, a powerful tentacle entity that relentlessly pursues Komachi. The humans and Jin are forced to confront these evolved, demi-human tentacles, which possess even greater cunning and malice than the common beasts. After a narrow escape, Komachi is guided by Dazune toward a facility housing a final trump card, a last resort to defeat the monstrous pursuer once and for all.