"Guy" is a science fiction anime series set in a dystopian future where humanity faces extinction due to the emergence of monstrous creatures known as "Hollows." These beings, born from human despair and negative emotions, wreak havoc across the world, leaving destruction in their wake. The story centers on Guy, a young man who becomes a "Vessel," a human host for a powerful entity called a "Guardian." These Guardians are ancient beings tasked with protecting humanity, but their immense power comes at a cost: the Vessels risk losing their humanity as they merge with their Guardians.

Guy's journey begins when he encounters a mysterious girl named Lina, who is revealed to be a key figure in the battle against the Hollows. Lina possesses the ability to purify Hollows, but her powers are unstable and require the protection of a Vessel. As Guy and Lina travel together, they uncover the truth behind the Hollows' origins and the role of the Guardians in humanity's survival. Along the way, they are joined by other Vessels, each with their own unique abilities and struggles, forming a fragile alliance to combat the growing threat.

The narrative explores themes of sacrifice, identity, and the blurred line between humanity and monstrosity. Guy grapples with the moral implications of his role as a Vessel, questioning whether the cost of wielding such power is worth the potential loss of his humanity. The series also delves into the backstories of supporting characters, revealing their motivations and the personal tragedies that led them to become Vessels. As the conflict escalates, the group faces increasingly powerful Hollows and internal tensions, testing their resolve and unity.

The climax of the series revolves around a final confrontation with the source of the Hollows, a being known as the "Void King," whose existence threatens to consume the world. Guy and his allies must confront their deepest fears and insecurities to stand against this ultimate threat, culminating in a battle that determines the fate of humanity. The series concludes with a bittersweet resolution, leaving room for interpretation regarding the future of the characters and the world they fought to save.