The False Kidnapping and Hostage Case

Situation
Kogoro and Megure are tasked with a critical case that involves the kidnapping of a girl named Naoko Takei, who is being held hostage in a van near a warehouse by the harbor. When the group plus Ran, Conan and Naoko's father Katsuhiko Takei approach the van, however, the kidnapper panics and drives the van into the sea itself, killing both Naoko and themself. It turns out Naoko was a classmate of Shinichi and Ran, and as Megure asks Ran about her, Conan thinks of how she was an awkward and quiet "lonely rich kid".

In the following days, Conan befriends a young woman named Akiko Hanai, who works as both Takei's secretary and Naoko's former caretaker. As he gathers information, he realises that Mr. Takei had more than one enemy thanks to his corrupt business tactics... and that there's a slim chance that Naoko may be still alive. But he has to be very fast and careful if he wants to uncover the truth and save her.

Resolution
Hide= The kidnapper is Akiko Hanai. 20 Years ago, Mr. Takei drove a fellow businessman to bankruptcy and then to insanity; he killed his wife and younger son Masahito and then himself, leaving his eldest daughter alone. She was adopted by her relatives, who changed her birth name; when she grew up, deciding to look for Takei and work for him under this new identity, to keep an eye on the man who ruined her life.
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Akiko kidnapped Naoko after an outing and then faked her own death as well as the girl's, to prove Takei's affection for his child. Naoko never was held hostage in the van: Akiko played the role of both the kidnapper/driver and the victim, and drove the van into the sea to fake her and Naoko's deaths. When they were speaking, Conan noticed that at some point Akiko pinched her nose in a strange manner -- to unclog her nose and ears after taking said "dive" and resurfacing too fast afterwards. He also saw that she was quite tanned save for her wrist, where she likely had a special wristwatch used by divers.

Later, Akiko contacts Takei and, still hiding her identity, tells him that Naoko is still alive, and that he should come alone to the same warehouse and bring five million dollars/yens if he wants to recover her. However, Takei uses false money as ransom; this enrages Akiko, and she decides to kill him as punishment via dousing him in gasoline and then gets ready to burn him to death. As she does so, she reveals her identity to Takei and adds that she would've released Naoko without even taking the ransom, had Takei cared enough for his child to use real money. Conan, who greatly reminds Akiko of the deceased Masahito, appears and talks her out from her murder plans.

Naoko is soon found unharmed by the police, and brought to the presence of Akiko and Takei. She ignores her father when he calls out to her and goes towards Akiko instead, giving her a hug and saying that now she knows her reasons, and forgives her for everything.

Trivia

 * Katsuhiko Takei Car Driver Name Mercedes-Benz 500 E [W124]
 * Akiko Hanai kidnapped car is a 1993 Toyota TownAce & Drive Mercedes-Benz 600 E [W124].