A cop dies in front of Ran and she forgets everything, including Conan. He has to solve the murders while watching the one person who knows him doesn't recognize him at all. The movie finds something brutal in that setup: Conan gets to save her, but she'll never know he was always the one saving her. Memory loss could be cheap plot mechanics here, but instead it becomes the cost of trauma, the thing that separates people who live through the same horror into different worlds. By the end, you're not watching a mystery get solved. You're watching a kid try to rebuild someone who's already gone, even though she's standing right there.
A string of targeted killings zeroes in on Tokyo's police force, each murder more brazen than the last. When a cop falls dead in front of Ran, the psychological trauma shatters her mind. She wakes with no recollection of her identity, her relationships, or the faces of those around her, including Conan. The killer notices she saw everything and realizes she's a loose end that needs tying up. Conan finds himself in an impossible position. He can't simply tell Ran about the murder or the danger chasing her, not when her fragile mental state might shatter further. Instead, he has to work the case from the shadows while keeping her safe from a murderer who grows more desperate with each passing moment. As Ran's memories remain locked away, the killer draws closer, forcing Conan to race against time to both solve the murders and restore what was taken from her.