"Gunkan Shōnen" is set in a world where humanity faces extinction due to the emergence of monstrous creatures known as "Kemono." These creatures, born from human desires and fears, wreak havoc on society, leaving destruction in their wake. The story centers on a young boy named Shiro, who possesses a unique ability to transform into a powerful battleship-like entity called a "Gunkan." This transformation is tied to his emotions and the mysterious "Core" embedded within him, which grants him immense strength but also puts him at risk of losing control.

Shiro is discovered by a secretive organization known as the "Guardians," who are dedicated to combating the Kemono threat. The Guardians recruit Shiro, recognizing his potential as a weapon against the Kemono. Alongside other Gunkan users, each with their own distinct abilities and Cores, Shiro undergoes rigorous training and missions to protect humanity. The group faces not only the external threat of the Kemono but also internal conflicts, as the origins and true nature of the Cores remain shrouded in mystery.

As the story progresses, Shiro grapples with the moral implications of his power and the sacrifices required to fight the Kemono. His relationships with fellow Gunkan users, particularly the stoic and experienced leader Kaito and the compassionate but conflicted Aoi, deepen as they confront increasingly dangerous foes. The narrative explores themes of identity, the cost of power, and the blurred line between humanity and monstrosity, as Shiro and his comrades uncover unsettling truths about the Kemono and their own existence.

The series balances intense action sequences with moments of introspection, as the characters navigate their roles in a world teetering on the brink of collapse. The Kemono, each with unique designs and abilities, serve as both physical and symbolic adversaries, challenging the protagonists to confront their own fears and desires. "Gunkan Shōnen" builds toward a climactic confrontation that tests the limits of Shiro's resolve and the bonds of his team, while leaving room for further exploration of its richly constructed world.