"Tide-Line Blue" is a 13-episode anime series set in a post-apocalyptic world where a catastrophic event known as the "Hammer of Eden" has flooded the Earth, leaving only scattered islands of land above water. The remnants of humanity struggle to survive in this new aquatic environment, with political and military factions vying for control over the limited resources. The story centers on two brothers, Keel and Aoi, whose lives take drastically different paths in this fractured world.

Keel, the elder brother, becomes the charismatic leader of a rebel group called "Neo-Atlantis," which seeks to overthrow the oppressive United Nations Navy (UNN) that governs the surviving population. Keel is driven by a desire to create a new world order free from the UNN's control, but his methods often blur the line between idealism and extremism. Aoi, the younger brother, is a carefree and somewhat aimless teenager who becomes inadvertently involved in the conflict when he encounters a mysterious girl named Isla, who holds the key to a powerful submarine called the "Ulysses." This submarine, a relic from the pre-apocalyptic era, becomes a focal point in the struggle between Neo-Atlantis and the UNN.

As the story unfolds, Aoi is thrust into the role of a reluctant hero, navigating the moral complexities of war and loyalty while grappling with his relationship with Keel. The brothers' conflicting ideologies and personal histories create a tense dynamic, with their bond tested by the escalating conflict. The series explores themes of survival, brotherhood, and the consequences of power, as both Keel and Aoi are forced to confront the realities of their choices.

Supporting characters include Isla, whose connection to the Ulysses and her own mysterious past play a pivotal role in the narrative, and Gonzo, a skilled pilot and loyal member of Neo-Atlantis who serves as a foil to Keel's idealism. The UNN, led by Admiral Gould, represents the authoritarian regime that Keel opposes, but the series also delves into the internal struggles and moral ambiguities within the organization.

The plot is driven by a series of naval battles, political intrigue, and personal conflicts, with the Ulysses serving as both a symbol of hope and a weapon of destruction. The series balances action with character-driven drama, culminating in a climactic confrontation that forces the characters to reckon with the consequences of their actions. "Tide-Line Blue" presents a world where survival is intertwined with sacrifice, and the line between heroism and villainy is often blurred.