"World Embryo" is a manga series that blends elements of science fiction, mystery, and action. The narrative centers around Riku Aiba, a young man who becomes entangled in a bizarre and dangerous phenomenon after the death of his girlfriend, Yuko. Following her passing, Riku discovers a mysterious tattoo-like mark on his chest, which grants him the ability to summon a powerful weapon known as a "World Embryo." This mark also connects him to a parallel world where shadowy creatures called "Shadows" prey on humans.

Riku learns that he is not alone in possessing this mark. Others, known as "Marked Ones," also bear the same symbol and are tasked with fighting the Shadows to protect humanity. These individuals are guided by enigmatic beings called "Angels," who provide cryptic instructions and oversee their battles. As Riku delves deeper into this hidden world, he encounters other Marked Ones, including a young girl named Ren, who becomes a key ally. Ren is a skilled fighter with her own World Embryo, and her presence helps Riku navigate the complexities of their shared mission.

The story explores the origins of the Shadows and the true purpose of the Marked Ones. It is revealed that the Shadows are manifestations of human despair and negative emotions, and the Marked Ones are part of a larger system designed to combat these threats. However, the Angels' motives remain ambiguous, and their relationship with the Marked Ones is fraught with tension and mistrust. As Riku and his allies uncover more about their powers and the world they inhabit, they begin to question the nature of their existence and the forces controlling their fate.

Throughout the series, Riku grapples with personal loss, the burden of his newfound responsibilities, and the moral dilemmas posed by his role as a Marked One. The narrative is driven by a combination of intense action sequences, psychological exploration, and intricate world-building. Themes of grief, identity, and the struggle against despair are woven into the plot, creating a layered and thought-provoking story. The manga concludes with a resolution that ties together the various threads of the narrative, offering closure to Riku's journey while leaving room for interpretation regarding the broader implications of the Marked Ones' existence.