"Case Closed: The Fist of Blue Sapphire" is the 23rd film in the "Case Closed" franchise, set in Singapore. The narrative centers around the theft of the legendary Blue Sapphire, a gemstone with historical ties to the city-state. The story begins with Kaito Kid, the infamous phantom thief, announcing his intent to steal the gem during a high-profile event at the Marina Bay Sands. Conan Edogawa, along with his friends Ran Mouri and Kogoro Mouri, travels to Singapore to investigate the case, only to find themselves entangled in a web of intrigue involving international criminals, political corruption, and a centuries-old maritime legend.

The Blue Sapphire is revealed to be connected to the historical figure Sang Nila Utama, the founder of Singapore, and is said to grant immense power to its possessor. As Conan and his allies delve deeper, they uncover a conspiracy orchestrated by a powerful organization seeking to exploit the gem's alleged abilities. The plot thickens with the introduction of Leon Lowe, a wealthy businessman with a hidden agenda, and his bodyguard, Reiko Starling, whose loyalties remain ambiguous. The narrative weaves together multiple subplots, including a rivalry between Kaito Kid and Conan, as well as the mysterious reappearance of Shinichi Kudo, Conan's true identity, which adds layers of tension and complexity.

The film features high-stakes action sequences, including a dramatic confrontation aboard a luxury cruise ship and a climactic showdown at the Marina Bay Sands SkyPark. Throughout the story, themes of trust, betrayal, and the pursuit of justice are explored, with Conan relying on his deductive skills to unravel the truth. The film also highlights the cultural and historical significance of Singapore, incorporating landmarks and local lore into the narrative. By the conclusion, the true motives behind the theft are revealed, and the characters must confront the consequences of their actions, leaving the audience with a resolution that ties together the various threads of the plot.