"Round Shell Second" is a manga series that centers on a post-apocalyptic world where humanity struggles to survive amidst environmental collapse and the rise of mysterious, shell-like creatures known as "Round Shells." These creatures, resembling large, armored mollusks, possess unique abilities tied to the planet's deteriorating ecosystem. The story unfolds in a fragmented society where remnants of human civilization are scattered across isolated settlements, each grappling with the scarcity of resources and the constant threat of the Round Shells.

The protagonist, a young scavenger named Kael, discovers a peculiar Round Shell with a humanoid form, which he names "Second." Unlike other Round Shells, Second exhibits sentience and the ability to communicate, forming a bond with Kael. This discovery sets off a chain of events as Kael and Second embark on a journey to uncover the origins of the Round Shells and their connection to the planet's ecological collapse. Along the way, they encounter various factions, including militarized groups seeking to exploit the Round Shells for their abilities and resistance movements fighting to restore balance to the environment.

The narrative explores themes of coexistence, environmental responsibility, and the consequences of human exploitation of natural resources. Kael's relationship with Second evolves as they face challenges that test their trust and understanding of each other. The series delves into the moral complexities of survival in a world where the line between humanity and nature has blurred, with the Round Shells serving as both a threat and a potential key to the planet's recovery.

Supporting characters include Lira, a scientist dedicated to studying the Round Shells, and Garret, a hardened mercenary with conflicting loyalties. Their interactions with Kael and Second add depth to the story, highlighting differing perspectives on the crisis and the role of the Round Shells. The plot builds toward a climactic confrontation as the true nature of the Round Shells is revealed, forcing Kael and his companions to make difficult choices that will determine the fate of their world.