The treasure hunt setup gives Conan permission to do what the series does best: watch him solve a problem while everyone else runs around having fun. The action sequences move like they actually cost money to animate. The mystery itself lands soft, it's not why you're here, but the movie knows that, and spends its runtime on the pleasure of watching a kid outwit grown men on an island instead of sweating whether the final reveal will stick the landing. The tone never pretends to be heavier than it is. For franchise fans, this is beach episode energy with actual stakes, not filler masquerading as plot. It's the weakest Detective Conan film made, but it's still Detective Conan doing what it does.

Conan and his friends arrive at Koumi Island for a vacation, but the getaway takes a turn when they encounter a group of treasure hunters on the island. The residents pass down an old legend: somewhere beneath the sea lies the Seabed Palace, an ancient ruin said to hold the lost treasure of Anne Bonny and Mary Read, two real female pirates from three centuries ago. Something about the hunters seems off, and while the Detective Boys go on a treasure search of their own, Conan comes across a murder. The police soon arrive, and the island falls into chaos as both hunters and locals race for the pirates' hoard while Conan, Kogoro and the police pursue the truth. The combination of historical intrigue, oceanic atmosphere and mystery creates an adventure that rewards those drawn to crime-solving with a side of swashbuckling excitement.