"Unlimited Fafnir" is a fantasy action anime set in a world where mysterious creatures known as Dragons suddenly appear, threatening humanity. These Dragons are not ordinary beasts; they possess immense power and are capable of transforming into human form. To combat this threat, the government establishes the Midgard Institute, a special academy where individuals with the ability to wield Dragon powers, known as Ds, are trained. The Ds are exclusively female, with one exception: Yuu Mononobe, the sole male D and the protagonist of the series.

Yuu transfers to Midgard and quickly becomes a central figure due to his unique status. The story explores his interactions with the female Ds, particularly the members of Brynhildr Class, a group of elite students tasked with defending humanity. Among them are Mitsuki Mononobe, Yuu's adoptive sister and the class representative; Tear Lightning, a skilled combatant with a stoic demeanor; and Iris Freyja, a cheerful and kind-hearted girl who forms a close bond with Yuu. As the series progresses, the group faces increasingly dangerous Dragons, each with its own abilities and motivations.

The narrative delves into the origins of the Dragons and the true nature of the Ds, revealing that the Dragons are not merely mindless monsters but beings with their own agendas. Yuu and his allies uncover a deeper conspiracy involving the government and the mysterious organization NIFL, which seeks to control the Dragons for its own purposes. The series balances action-packed battles with moments of character development, exploring themes of identity, trust, and the consequences of wielding immense power.

As the story unfolds, Yuu and his companions confront powerful adversaries, including the Black Dragon, a formidable foe with a personal connection to Yuu. The series builds toward a climactic showdown that tests the bonds between the characters and their resolve to protect humanity. Through its blend of supernatural elements, interpersonal drama, and high-stakes conflict, "Unlimited Fafnir" presents a compelling narrative centered on the struggle against overwhelming odds and the search for understanding in a world filled with chaos.