"For Immortal Children in the Galaxy" is a science fiction manga that explores themes of immortality, identity, and the human condition within a sprawling interstellar setting. The narrative centers on a group of children who have been genetically modified to achieve immortality, rendering them immune to aging and disease. These children, known as the "Eternals," are the result of a clandestine experiment conducted by a powerful intergalactic corporation seeking to unlock the secrets of eternal life. However, the experiment's success comes at a cost: the children are unable to reproduce and are destined to outlive everyone they know, leaving them isolated and grappling with the psychological burden of their condition.

The story begins with the Eternals being scattered across the galaxy after the corporation's facility is destroyed in a rebellion. Each child possesses unique abilities tied to their genetic modifications, ranging from enhanced physical strength to telepathic communication. The protagonist, a young girl named Lira, emerges as a central figure. Lira's ability to manipulate time on a small scale becomes crucial as she seeks to reunite the scattered Eternals and uncover the truth behind their creation. Along the way, she encounters other key characters, including Kael, a boy with regenerative powers who struggles with the moral implications of their immortality, and Sera, a telepath who serves as the group's emotional anchor.

As the Eternals journey through various planets and star systems, they face numerous challenges, including hostile alien species, rogue AI, and remnants of the corporation that created them. The narrative delves into the ethical dilemmas surrounding immortality, such as the loss of purpose over infinite time and the societal implications of a group that cannot die. The children also confront their own identities, questioning whether they are truly human or something entirely different. Their interactions with mortal beings further highlight the divide between their existence and the natural life cycle of the universe.

The plot thickens as the Eternals discover that their creation was not merely a scientific endeavor but part of a larger plan to combat an impending cosmic threat. This revelation forces them to reconcile their personal struggles with a greater responsibility to protect the galaxy. The manga balances action-packed sequences with introspective moments, creating a narrative that is both thrilling and thought-provoking. By the end of the series, the Eternals must decide whether to embrace their role as guardians of the galaxy or seek a way to relinquish their immortality and reclaim their humanity.