Three mysterious humanoid monsters known as Bem, Bela, and Belo arrive in a large, bustling coastal city. They immediately sense a corrupt and evil atmosphere hanging over the metropolis, a darkness brought about by immoral human behavior and the mischief of other monsters and yokai. Deciding to stay, they dedicate themselves to protecting the city's human populace from these supernatural threats. Despite their heroic efforts, the three are often met with fear, abuse, and discrimination due to their frightening appearances. Their deepest wish is to be rewarded for their good deeds by finally becoming human beings.

The trio consists of Bem, the tall and dark-clad leader who is serious and carries a cane; Bela, the sole female of the group who acts as caretaker and wields a whip-like weapon; and Belo, the youngest and most childlike member who is energetic and eager to make friends. In the 2006 remake series, they befriend a young girl named Kira Hinata, who sees past Belo's monstrous form and becomes his first true human friend, alongside other children like Kaito Kura and Miki Kisaragi. Their origins are mysterious, with the series suggesting they were born from a single multiplying cell in a dark, silent place, creating beings different from both humans and other life forms.

The narrative typically follows an episodic structure where the trio encounters a specific monster or supernatural crime each episode, ranging from creatures like the Headless Horseman Dullahan to a cursed painting that devours souls. As they fight to protect humans who often reject them, they struggle with their own isolation and the hope that their accumulated good deeds will one day transform them. This core conflict continues across the franchise's iterations, including a 2006 remake that modernizes the setting and a 2011 live-action television series where they disguise themselves as a human family while pursuing the same centuries-old dream of humanity.