"Unlimited Fafnir" is a manga adaptation of the light novel series written by Tsukasa and illustrated by Korie Riko. The story is set in a world where mysterious creatures known as Dragons suddenly appear, wreaking havoc and threatening humanity. These Dragons are not ordinary beasts; they possess immense power and are nearly invincible. In response to this threat, a unique phenomenon occurs: certain girls, known as "D," begin to manifest powers derived from the Dragons themselves. These girls are the only ones capable of combating the Dragons, as they can generate weapons and abilities tied to their Dragon heritage.

The protagonist, Yuu Mononobe, is a rare male "D" who possesses the same powers as the girls. After a tragic incident involving his younger sister, Mitsuki, Yuu is separated from her and taken into a secret military organization. Years later, he reunites with Mitsuki at Midgard, a special academy established to train and protect the "D" girls. Mitsuki, now the student council president, is overjoyed to see her brother again, but their reunion is complicated by the presence of other "D" girls, each with their own unique abilities and personalities. Among them are Iris Freyja, a cheerful and kind-hearted girl with a mysterious connection to the Dragons, and Tear Lightning, a stoic and skilled combatant.

As Yuu integrates into life at Midgard, he discovers that the Dragons are not merely mindless destroyers but are driven by a deeper purpose. The "D" girls, including Yuu, are linked to the Dragons through a phenomenon called "mateki," which grants them their powers but also binds them to the Dragons' will. The story explores the growing bonds between Yuu and the girls as they work together to uncover the truth behind the Dragons' existence and their own powers. Along the way, they face numerous challenges, including internal conflicts, external threats, and the moral dilemmas of wielding such destructive abilities.

The narrative delves into themes of identity, sacrifice, and the consequences of power, as the characters grapple with their roles as both protectors and potential weapons. The Dragons' true intentions remain shrouded in mystery, and as the story progresses, Yuu and his companions uncover startling revelations about their origins and the world they inhabit. The manga balances action-packed battles with moments of emotional depth, as the characters' relationships evolve and their understanding of the Dragons deepens. Through its exploration of the bonds between humans and Dragons, "Unlimited Fafnir" presents a compelling tale of survival, camaraderie, and the search for meaning in a world teetering on the brink of destruction.