Tanpopo Ohanami was only a child when her parents were killed by a fiend, a monstrous creature that feeds on human life force. Her survival was owed to Shade, a powerful shadow fiend who defeated her parents' killers and took her under his wing. Since that day, Shade has secretly protected Tanpopo from other fiends and from her own occasional missteps, becoming her closest and most unusual best friend.

Now sixteen years old, Tanpopo lives a relatively normal life as a clumsy but energetic high school student, all while keeping the serrated-toothed secret of her friendship with Shade. In this world, the only beings capable of destroying fiends are magical girls, warriors who serve under a special anti-fiend organization known as Butterfly. When Butterfly recruits Tanpopo, citing her extraordinary magical potential, she is ecstatic at the chance to join the fight against the creatures that have caused so much suffering. She dreams of finally being able to fight alongside Shade, who has been eliminating his own kind in secret.

Shade is vehemently opposed to Tanpopo becoming a magical girl, warning her that their friendship will force them to become enemies should she accept. Despite his grumpy and sarcastic exterior, he harbors a genuinely kind and caring nature, making him a clear example of dark not being evil in a world that treats fiends as irredeemable villains. Tanpopo inadvertently awakens her powers, but as she begins to navigate her new role under the guidance of Peony, a graceful and masterful senior magical girl who wields a katana, she quickly learns that the truth about both fiends and magical girls is far more complicated than she ever imagined.

The initial narrative arc focuses on Tanpopo's difficult transition and the strain on her relationship with Shade. A major crisis erupts when Tanpopo's own grandfather falls victim to fiendhood, forcing her to confront the personal cost of the conflict and setting the stage for a deeper mystery. As the story unfolds, it explores the dark history of the organization Butterfly, the secrets Shade carries from his past, and the true nature of the bond between a magical girl and the one creature she was never supposed to call a friend.