Three humanoid monsters named Bem, Bela, and Belo live in disguise among humans in a coastal city plagued by crime and supernatural threats. Born from a single dividing cell in murky silence, they are neither human nor beast, possessing hideous true forms that emerge when their emotions spiral out of control due to intense anger or sorrow. Their deepest wish is to become human someday, believing that their good deeds in protecting people from evil creatures might one day grant their desire. Despite their noble sense of justice, humans fear and hate them solely for their monstrous appearances, forcing the trio to hide their identities while continuing their mission.

Bem is the oldest and acts as the group's leader, carrying a strong sense of justice and the ability to sense evil energy anywhere in the city. He appears muscular, intelligent, and quiet, seemingly emotionless but deeply caring for Bela and Belo as his family. Bela is the only female among them, temperamental and expressive, though she often suppresses her talkative nature around her reserved companions. She wields a whip built into her arm and possesses the ability to generate freezing cold air. Belo is the youngest, a genius gamer who spends much of his time in arcades, wearing headphones to keep distance from human society. He has doubts about humanity and a cool, sarcastic personality that masks his kind heart.

The 2011 live-action television drama, which aired on NTV beginning October 22, follows this trio as they encounter Detective Natsume, a human who becomes involved in their quest. After Bem saves a man, he befriends Natsume, who possesses a photograph that may hold a clue about how the monsters can become human. Their investigation leads them to a man named Ogata, who denies any knowledge, disappointing the trio. Meanwhile, the case involving the man Bem rescued unfolds rapidly, placing Natsume in danger and forcing the humanoid monsters to intervene.

A separate live-action series focused specifically on Bela, titled Humanoid Monster Bella Episode 0, was released in 2020 consisting of 10 episodes. This narrative centers on Umenokisaka Bella, a transfer student who arrives in the class of Ohno Shiori. Her classmates initially show interest in her striking appearance, but when she fails to respond to their attention, they resort to bullying. As the episodes progress, Bella becomes entangled in the lives of her classmates, each struggling with personal demons. One student is forced into theft at night, another obsesses over social media followers, an idol wants to retire from her career against her mother's wishes, and a class leader cracks under her father's academic expectations. Shiori watches as Bella systematically dismantles each of these classmates, one after another, and resolves to uncover the truth behind Bella's connection to a deceased student named Kozue. The series culminates in a confrontation where the true nature of Bella is revealed, forcing her to fulfill a promise made to Kozue.

The live-action continuity also includes a 2012 feature film released on December 15, continuing the story of Bem, Bela, and Belo after they disappear from their human friends. Roaming the world, they arrive at a town terrorized by bizarre crimes, where they confront a nemesis whose identity leads to a horrible secret and the possibility that they might finally achieve their dream of becoming human, though their very existence hangs in the balance. Across all iterations, the central theme remains consistent: beings who fight for humanity while never being accepted by it, longing for a transformation that always seems just out of reach.