"Pikaia!!" is a science fiction anime series set in a distant future where humanity has evolved into a new form of life known as the Pikaia. The story unfolds in a world where Earth has undergone drastic environmental changes, forcing humans to adapt biologically and technologically. The Pikaia are a hybrid species, combining human intelligence with aquatic traits, enabling them to thrive in the planet's now predominantly water-covered surface.

The narrative centers on a young Pikaia named Aoi, who discovers a hidden ability to communicate with ancient marine creatures thought to be extinct. This ability draws the attention of the ruling Council of Pikaia, a governing body that seeks to maintain control over the population by suppressing knowledge of the past. Aoi's journey begins when they encounter a rogue scientist, Dr. Kaida, who reveals that the Council has been concealing the truth about humanity's origins and the catastrophic event that led to the current state of the world.

As Aoi and Dr. Kaida delve deeper into the mysteries of the past, they uncover a network of underground resistance fighters known as the Abyssal Order. This group aims to restore the lost history of humanity and challenge the Council's authoritarian rule. Along the way, Aoi forms alliances with other Pikaia, including Ren, a skilled warrior with a personal vendetta against the Council, and Sora, a tech-savvy inventor who provides crucial tools for their mission.

The plot thickens as the group discovers an ancient artifact capable of reversing the environmental damage done to Earth. However, the Council, led by the enigmatic Chairman Ryuu, views the artifact as a threat to their power and mobilizes their forces to seize it. Aoi and their companions must navigate treacherous waters, both literal and metaphorical, as they face off against the Council's advanced technology and formidable enforcers.

The series explores themes of identity, environmental responsibility, and the consequences of unchecked power. Aoi's internal struggle with their dual nature as both human and Pikaia mirrors the broader conflict between preserving the past and embracing the future. The story builds toward a climactic confrontation where Aoi must decide whether to use the artifact to restore Earth's former state or to forge a new path for the Pikaia species.

"Pikaia!!" combines elements of adventure, mystery, and political intrigue, creating a compelling narrative that examines the complexities of evolution and the enduring human spirit. The characters' growth and the unfolding revelations about the world's history drive the plot forward, making it a thought-provoking and engaging series.