Diplomat Murder Case

Part 1
Conan has to deal with some stiff competition when Heiji Hattori, heralded as the "Great Detective of the West", comes looking for Shinichi Kudo to prove which of them is the best detective. He shows up all of a sudden in the Mouri Agency (since he suspects Shinichi has a hand on Kogoro's recent success and Sonoko told him where it was located), bringing a nice Osakan liquor as a gift to Kogoro (so he'll let him stay overnight), and delivers the challenge, which will take place regarding the request coming from a rich woman: to make a background check on her future daughter-in-law, a young woman who's simply too good to be true so she could actually be a bitch in sheep's clothing or a gold digger in disguise.

However, before Conan can save his reputation, he must first save himself, as a mysterious sickness threatens to close his file for good... and then check on his surroundings, as the husband of their client appears dead right after the group arrives to his big and fancy house!

Part 2
Heiji Hattori believes that the murder of the diplomat Isao Tsujimura is a locked room one, and he's also sure of who the killer is. But almost at the same time, Conan collapses due to his illness... and then Shinichi Kudo returns to the surprise of everyone, especially himself! As Shinichi tries to expose the real secret behind the perfectly sealed room, the rivalry between he and Heiji Hattori heats up! Deceit and treachery are revealed, as Isao Tsujimura's killer is finally unmasked and their reasons to commit the crime are explained!

Resolution
Hide= The Killer is Kimie Tsujimura. She murdered her husband by first drugging him with sleeping pills before going to the Mouri agency, then pretending to try waking him up... but actually poisoning him with a poison-embibed needle that she hid inside a locked attached to her keychain. (Which looked exactly like a locket/keychain that Tsujimura himself had). She also tried to stage a locked room mystery with Tsujimua's locket keychain and several fishing line threads specifically placed all around the home, to make everyone believe that Tsujimura had died thanks to a trick that was triggered right when they opened the door to his studio; in reality he was alive but asleep due to the drugs, so Kimie secretly prickled him to his death in front of everyone.
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Kimie's reasons to murder her husband were the following: twenty years before she was was happily married to a man named Kenji Yamashiro and they had a little daughter, Yukiko. Tsujimura, who was in love with Kimie and jealous of Yamashiro, staged a cruel trap to accuse him of being involved in a fraud; Yamashiro was sent to jail and died there soon, then Tsujimura courted the extremely distraught Kimie, and as soon as she was widowed he divorced his wife/Takayoshi's mother and convinced Kimie to marry him. Kimie was unaware of this and accepted, since she was completely devastated and had to give up her child's custody to Yamashiro's family.

Twenty years after the "fraud", the adult Takayoshi brought a picture of a girl he had fallen for and intended to marry: she happened to be Yukiko, Kimie and Yamashiro's daughter, who had grown to look almost exactly like Kimie save for her hair/color scheme. (As seen in an old picture of the just-married Kimie and Tsujimura, which the latter kept in his studio). Tsujimura realised who the girl was, was completely horrified and started screaming at his son for wanting to marry "that man's child". Kimie also recognized Yukiko in the picture and started to get suspicious, then Tsujimura revealed the truth to her. So she decided to kill her "husband" to get at him, using her visit to Kogoro to ask about Yukiko's background as an excuse that would provide her for an alibi.

(As a corollary, Toshimitsu Tsujimura had helped his son Isao in his plan to "steal" Kimie from Yamashiro and send him to jail. He had repented long ago, though, and he wanted to take the blame for his son's murder as a way to atone for his terrible sin; therefore, when Heiji said that he was the culprit, he willingly confessed for a crim he hadn't commited, trying to wash away for his part in ruining the Yamashiro family's lives.)

Kimie recognizes that she is the killer, reveals the above mentioned motivations and surrenders to the police without any resistence. Before being taken away, she begs Takayoshi to take good care of her daughter Yukiko, and Takayoshi promises to do so as Yukiko cries for all of this.

Heiji aknowledges his loss and tells this to Shinichi, aknowledging him officially as the winner of this case. Shinichi, however, calls him a fool and says that it's useless to compete, since there is only one truth ever. After saying this, he almost collapses atop of the stairs; since he had recovered his original physical age solely as an effect of the homemade liquor (doubling as a folk cure for colds) that Heiji made him drink earlier and the cold he had, it's only temporal and it will wear off soon.

Ran,, who logically knows nothing of this, catches him trying to sneak away and tries to make him stay so he will go to a doctor... He tells her to wait and gets away by practically throwing himself down the stairs as his body begins to actually reverse to a child's one, nevermind his mental pleas to resist a little so he can stay with Ran a little longer. Then, Shinichi almost screams Ran's name before fully shrinking back.

When Ran finally catches up, she finds Conan alone in an oversized shirt. He tells her that Shinichi has just left and then passes out of pain and light-headness, which keeps Ran from making him any more questions.

Heiji returns to Osaka and, once Conan gets mildly better, he wonders why did his body return to normal even if for some minutes. He realises it was an after-effect of the liquor... but Ran catches him trying to snatch the bottle from the drunk Kogoro and takes it away. Conan is upset but then the Detective Kids drop by to visit, which cheers him up a little.