Milly, a soldier from the year 2084, is caught in a desperate battle against an alien race known as the "Daggra," which has pushed humanity to the brink of extinction. In a last-ditch effort to save her people while holed up in the final bastion in Tibet, she jumps into a newly constructed time portal just as the fortress is breached. This leap catapults her to 2002, where her mission is clear: eliminate the first Daggra, who has fabricated a crash landing to signal an invasion fleet.
Upon her arrival, Milly stumbles into the chaotic aftermath of a shootout in Tokyo Bay, confronting the hitman Miyamoto who has the notorious Triad mobster Mizoguchi at gunpoint. In a twist of fate, her sudden appearance becomes a window of opportunity for Mizoguchi to escape, prompting Miyamoto to take Milly—whom he initially believes he has inadvertently injured—back to his home. Fortunately, a metal plate in her coat saves her life. To coerce Miyamoto into aiding her, she meticulously tapes a bomb to his neck, setting the stage for a reluctant alliance driven by vengeance, as Miyamoto has his own score to settle with Mizoguchi, who had cruelly murdered his childhood friend.
That evening, as Milly tidies Miyamoto's trenchcoat, he urges her to rest. However, the quiet is shattered the following morning when he discovers photographs and a newspaper clipping about his own demise. Confused, he consults his weapon supplier Shi, who dismisses the notion that the Triads would invest time in such elaborate deceptions.
With Shi's resources, they track the alien's crashed spaceship, only to discover it's been transported to the National Institute of Space Science. As they attempt to infiltrate the facility, their plans are thwarted when Mizoguchi arrives, intent on seizing control. Milly's expectations are shattered upon confronting the alien, which isn't the vile creature she anticipated. As Mizoguchi enters the lab, Milly hesitates to pull the trigger, only to learn that humanity, not the aliens, is responsible for the ongoing war—they had captured and killed the alien first. It becomes clear to her that stopping Mizoguchi is essential, as he seeks the alien's technology to dominate the planet.
In a chaotic turn of events, after Mizoguchi and his gang take the alien and its ship from the ruined lab, Milly and Miyamoto regroup for their next encounter. Their confrontation at a deserted oil rig unfolds dramatically as they manage to rescue the alien. Amid a sweeping explosion and a bloodied Mizoguchi threatening retribution, his gunfire meets an unforeseen force field. Seizing the moment, Miyamoto snatches the weapon from Mizoguchi's grip and decisively ends his reign. Just as they try to comprehend the mysterious force field, a Daggra craft, masquerading as a commercial airliner, appears on the scene—responding to the alien's distress call. The Daggra whisk away their injured comrade, signaling the collapse of the impending war, leading to Milly's gradual disappearance.
In the aftermath, Miyamoto resolves to leave behind the violent life he knew, surrendering his weapons to Shi. While making his way home, he inadvertently crosses paths with a thug whose life he had previously spared at Tokyo Bay. Defenseless and unarmed, Miyamoto finds himself at the mercy of the thug, who shoots him, presuming he has met his end. To his surprise, he soon finds a familiar metal plate—just like the one that had saved Milly—has absorbed the bullet. Inscribed on it is Milly's message of gratitude, implying she had fulfilled her debt to him. In a poignant reflection, Miyamoto recalls the previous night when Milly had playfully handled his trenchcoat. Unknown to him, a future version of Milly had journeyed back in time, slipping the protective plate into his coat before returning to her own timeline, leaving behind a newspaper detailing the news of his death.
