Kanisuke Akigai is an ordinary high school student living in a world where demi-humans, people with animal characteristics, are increasingly common. His settled life is upended when a strange and cute transfer student named Yuuko Mizuda, nicknamed Mizudako-chan, joins his class. Yuuko is a demi-human with two large, powerful octopus tentacles growing from her head in place of hair. She immediately takes an intense and inexplicable interest in Kanisuke, frequently making unsettling declarations that he will never escape from her.

The story follows the slow-burning and weird romance between Kanisuke and the creepy-cute Yuuko. As they bond, Kanisuke learns that Yuuko is deeply lonely, having been shunned her whole life because of her intimidating appearance and an uncontrollable power: when stressed or emotional, she can transform the heads of those around her into sea creatures. While this ability frightens most people, Kanisuke reacts with fascination rather than fear, which makes Yuuko incredibly happy. It is gradually revealed that Yuuko has been searching for Kanisuke for years, implying a significant connection from their childhood that he does not fully remember.

The narrative focuses on their everyday school life and Kanisuke helping Yuuko adjust to human society. Her tentacles create a recurring source of minor conflicts, such as drooping over her ears when she is tired, making her appear aloof when she simply cannot hear anyone. Despite her sinister side, Yuuko performs many acts of kindness for Kanisuke, which their classmates silently cheer on. The first major arc introduces the world, establishes Yuukos stalker-like devotion, and confirms their past connection. Later chapters introduce side characters including Iwane, an energetic tsundere girl who wants to befriend Yuuko, and Nakayama, a perverted class clown. A significant early story beat involves the appearance of a Nekopus, a cat with an octopus for a head, who is secretly a family member checking on Yuuko to ensure her powers do not cause trouble.