"Lupin the 3rd: Blood Seal of the Eternal Mermaid" is a feature-length anime special that centers on the exploits of the master thief Arsène Lupin III and his team. The narrative begins with Lupin and his partner Daisuke Jigen infiltrating a high-security auction to steal a mysterious artifact known as the "Eternal Mermaid." This artifact is said to grant immortality, making it a highly coveted item among collectors and criminals alike. During the heist, Lupin encounters a young woman named Meru, who claims to be a mermaid and is pursued by a shadowy organization seeking to exploit her unique abilities.

The story unfolds as Lupin, Jigen, and their associates—Fujiko Mine and Goemon Ishikawa—become entangled in a dangerous conspiracy involving Meru and the artifact. The group is pursued by Inspector Zenigata, who is determined to capture Lupin, as well as by the enigmatic organization that seeks to harness Meru's powers for their own nefarious purposes. As the plot progresses, it is revealed that Meru's connection to the Eternal Mermaid artifact is deeper than initially thought, and her existence is tied to a centuries-old secret involving immortality and betrayal.

The narrative is driven by a series of action-packed sequences, including car chases, gunfights, and elaborate heists, as Lupin and his team navigate a web of intrigue and deception. The story explores themes of trust, loyalty, and the consequences of seeking eternal life, while maintaining the signature blend of humor, suspense, and adventure characteristic of the Lupin the 3rd franchise. The special also delves into the dynamics between the main characters, highlighting their individual strengths and the camaraderie that defines their partnership.

Ultimately, the resolution of the story hinges on Lupin's ability to outwit his adversaries and protect Meru from those who would exploit her. The climax involves a dramatic confrontation that tests the limits of Lupin's cunning and resourcefulness, culminating in a conclusion that ties together the various threads of the plot while leaving room for the enduring legacy of Lupin's adventures.