In the midst of German-occupied France, a dedicated archaeologist, Professor Bresson, faces grave danger as he entrusts his heavily booby-trapped diary and a mystical amulet—integral to unlocking the secrets of the fabled treasure known as the "Eclipse"—to his family. This invaluable diary holds the potential to lead its readers to untold riches, coveted by a nefarious group of Nazis aligned with the Ahnenerbe organization. Unfortunately, the sinister forces arrive just in time, leading to Bresson's untimely death at the hands of German soldiers. Although his family manages to escape, they find themselves pursued by the relentless Professor Lambert, resulting in a catastrophic car crash that leaves only Bresson's infant daughter as the lone survivor. In a cruel twist of fate, Lambert seizes the amulet but fails to locate the diary, which has been misplaced during their harrowing escape.
Fast forward to the 1960s, when the diary miraculously resurfaces during a memorial exhibition dedicated to Bresson. The renowned thief, Lupin III, makes a bold attempt to steal the diary, following in the footsteps of his grandfather, who had tried to achieve the same goal many years earlier. However, his efforts are thwarted first by a clever young woman posing as a security guard and then by the cunning Fujiko, ultimately leading to his arrest by the vigilant Inspector Zenigata.
As Lupin is being transported to prison, he is rescued by his loyal friends, Jigen and Goemon, and makes the startling decision to infiltrate the home of his earlier adversary, the aspiring archaeology student Laetitia Lambert. During this encounter, Lupin reveals a medal identical to the one taken by Lambert—a precious heirloom left to him by his grandfather. This connection prompts Laetitia to reach out to Lambert, her adoptive grandfather, and the very man who orchestrated her theft of the diary, under the false pretext of assisting her education at Boston University.
In an unexpected twist, Fujiko delivers the diary to a group of Ahnenerbe fugitives, including Lambert and the merciless leader, Geralt. However, Geralt soon uncovers Fujiko's betrayal, leading to her unexpected capture. Undeterred, Laetitia guides Lupin to Lambert's secret hideout—a customized transport plane, where they secure the prized diary along with the complementary amulet that Lupin possesses. Upon examining the diary, they discover that the Eclipse is an advanced power generator left behind by a long-lost civilization, hidden within the depths of the ancient ruins of Teotihuacan. Aware that he’s been led into a clever trap, Lupin surrenders to Geralt and Lambert. Meanwhile, Fujiko cleverly uses Lupin as a diversion, managing to escape to safety.
Utilizing a radio, Lupin taunts Zenigata to find him, ultimately stealing his Interpol helicopter amidst the chaos. Realizing the gravity of the situation, Zenigata reluctantly teams up with Lupin, Laetitia, and Jigen to thwart the malevolent plans of Geralt and Lambert. During a critical moment of revelation, Lupin discloses to Laetitia that he deduced her true identity as Bresson's granddaughter and exposes Lambert's intentions of adapting her for his own profit. He also learns that his own grandfather had once aided Bresson in discovering the Eclipse, but had deliberately concealed the diary to protect the treasure from falling into perilous hands.
As events unfold, Geralt and Lambert locate the hidden Eclipse, yet they find themselves hindered by the necessary diary. Seizing the opportunity, Lupin and his team strive to advance through the perilous obstacles, only to learn that their foes’ retreat was a strategic ruse. Unfortunately, the malevolent duo of Lambert and Geralt successfully activate the Eclipse, taking Laetitia along with them. However, Lupin and his allies are quick to intercept the plane before they can escape, resulting in intense confrontation. In a critical flash of brilliance, Laetitia seizes control of the powerful device just as Geralt attempts to eliminate her, fatally injuring Lambert in the process.
As the stakes elevate dramatically, Geralt receives a shocking update revealing that Hitler has been located, leading him to commandeer the Eclipse, Laetitia in tow. Meanwhile, as tensions rise, Laetitia discovers that Lupin’s team, with the backing of Zenigata and Interpol, have arrived at the Ahnenerbe base ahead of Geralt and have prepared for a final showdown. In a daring maneuver, Lupin disguises himself as Hitler aboard the Eclipse, only to reveal his true identity when cornered by Geralt. In the ensuing chaos, Lupin sabotages the Eclipse's controls, unleashing a devastating micro black hole that claims Geralt's life.
In the aftermath, Lupin makes a quick escape, ensuring Laetitia receives a gravity device as a parting gift, along with her enrollment in Boston University as promised. With a final goodbye, he and his crew hastily evade the pursuit of Zenigata, leaving behind a trail of adventure and intrigue.
