"Pikaia!" is a science fiction anime series set in a distant future where humanity has colonized multiple planets across the galaxy. The narrative centers on a young protagonist named Kaito, a skilled pilot and engineer who becomes embroiled in a conflict between Earth's governing body, the United Planetary Alliance (UPA), and a rogue faction known as the Nebula Vanguard. The Nebula Vanguard seeks to overthrow the UPA, claiming it has become corrupt and oppressive, while the UPA frames the faction as terrorists threatening galactic stability.

Kaito's journey begins when he discovers a mysterious alien artifact during a routine mission on a remote planet. This artifact, later revealed to be a fragment of an ancient civilization's technology, grants him access to a powerful mecha named Pikaia. The mecha is uniquely sentient, capable of forming a symbiotic bond with its pilot, and possesses abilities far beyond conventional human-made machines. As Kaito learns to harness Pikaia's power, he becomes a key figure in the escalating conflict, drawing the attention of both the UPA and the Nebula Vanguard.

The series explores themes of loyalty, morality, and the consequences of technological advancement. Kaito is joined by a diverse cast of characters, including Aria, a former UPA officer disillusioned with the government's methods, and Zane, a skilled hacker with ties to the Nebula Vanguard. Together, they navigate the complexities of the war, uncovering secrets about the artifact, the ancient civilization that created it, and the true motives of both the UPA and the Nebula Vanguard.

As the story progresses, Kaito and his allies face increasingly difficult choices, balancing their personal ideals with the greater good. The narrative builds toward a climactic confrontation that challenges the characters' beliefs and reshapes the future of the galaxy. The series concludes with an open-ended resolution, leaving room for potential exploration of its richly constructed universe in future installments.