Spoilers: This article covers the poisoning solution, the motive and the explanation of all three couples’ apparent infidelity.
Waitress Aijima poisons Nagasu by arranging for her to handle a contaminated chair before eating. The Suspected Infidelity of the Triple Collab (Part 1) and The Suspected Infidelity of the Triple Collab (Part 2) are international episodes 1188–1189 (Japanese episodes 1130–1131), adapting Files 1091–1093 across Volumes 102–103.
The dessert is the point at which the poison is swallowed, not the object originally prepared with it. Meanwhile, the three unusual pairings that alarm the Detective Boys are misunderstandings: Chiba and Naeko, Shiratori and Kobayashi, and Shukichi and Yumi remain the actual couples.
A restaurant visit produces three romantic puzzles
After a film, the Detective Boys and Professor Agasa go to Danny’s and talk about the popular band the Night Hearts. Three fans—Nagasu, Toya and Kokonoe—join the conversation. Their friend Aijima is working as a waitress and seats them where they can enjoy the night view.
Elsewhere in the restaurant, Ayumi, Genta and Mitsuhiko spot familiar adults and expect to tease them about their dates. Instead, they find Sumiko Kobayashi with Kazunobu Chiba, Shukichi Haneda with Naeko Miike, and Ninzaburo Shiratori with Yumi Miyamoto.
Those are recognizably the wrong partners. Haibara becomes particularly interested in investigating the apparent infidelity, and even Agasa is drawn into the children’s curiosity. The comedy comes from their confidence that a visible pairing must explain the relationship behind it.
The band discussion belongs to both strands of the episode. It supplies a shared interest for the restaurant’s customers and later becomes part of the motive for murder, while also connecting the couples’ much more innocent plans.
A birthday blackout precedes the poisoning
Nagasu, a twenty-nine-year-old pet-shop employee, habitually appears on the right in photographs with her friends. After the group takes pictures, she also settles naturally into the right-hand seat. Aijima has anticipated that preference in choosing where to place them.
The restaurant darkens for a birthday surprise. During the interruption, Nagasu deals secretly with a chair she believes she has damaged. Afterward, the friends sample maritozzi, and she collapses from cyanide poisoning.
The emergency brings Chiba, Shiratori, Shukichi and the three women together. The discovery of their unusual combinations makes an already serious scene painfully awkward. Their private explanations have to wait while the police deal with the death.
Shiratori sends Conan and the children out to wait in the parking area. Haibara’s outspoken interest in the romantic situation has not helped them remain welcome. This changes how Conan can contribute: he must work through people who can still return to the scene.
The killer uses a seating habit and a moment of embarrassment
Aijima has deliberately made the chair intended for Nagasu unstable and applied poison to a part Nagasu will touch when moving it. The adjacent reserved table supplies an available replacement. The waitress’s ordinary authority over seating allows her to bring those conditions together without appearing to do anything extraordinary.
Nagasu assumes her weight has damaged the wobbly chair. When the birthday lights go out, she uses the privacy of the darkness to exchange it with a chair from the reserved table. In handling the furniture quietly, she transfers the poison to her hand.
She then eats the dessert with that hand. The attack therefore does not require Aijima to poison an individual pastry in front of the group. By responding predictably to an embarrassing inconvenience, Nagasu carries the contaminant from the prepared chair to the food herself.
The blackout is not merely cover for the culprit to move. It is an opportunity for the victim to perform the action the culprit expects. The right-hand seating preference, loose chair and nearby reserved table make that opportunity specific to Nagasu.
Photos and camera footage return Conan to the case
The group photographs reveal Nagasu’s consistent position, while security footage and the chair exchange allow Conan to reconstruct the route of contact. The question shifts from who could have put poison in a pastry to who could have arranged for Nagasu to touch it elsewhere.
Kobayashi proposes asking Conan for help. Yumi hears his explanation over the phone and decides to present the deduction herself, with Naeko enthusiastically joining in and Kobayashi drawn into the presentation. The women who were being watched as possible unfaithful partners become the route by which the solution reaches the room.
They place the suspects at the seats and use the dessert in a reconstruction. Aijima alone cannot bring herself to eat it. Her reaction matters alongside the physical explanation: someone who knows where the poison has been placed has a reason to fear a sequence that seems harmless to the others.
That response is not the entire case. It supports the combined evidence of the photos, the seating she selected, the unstable chair, the blackout and the recorded movements. Her role as waitress provides the opportunity to arrange the conditions that all point in the same direction.
Aijima blames Nagasu for the loss of her boyfriend
Aijima had been dating Shibami, a former member of the Night Hearts. Nagasu told another female fan where he regularly went, leading to a photograph of the couple and public exposure of their relationship.
The resulting backlash was followed by Shibami leaving the band, drinking heavily and dying in an accident. Aijima sees Nagasu’s disclosure as the beginning of the loss that destroyed her life. She also resents Toya and Kokonoe because they were present and did not stop it.
The murder victim is Nagasu; the other two friends’ presence in Aijima’s grievance does not mean that all three were killed. The plan described here targets Nagasu’s habit and her seat.
The motive explains why a conversation about a favorite band becomes so painful in retrospect. It does not make Nagasu’s death a remedy for Shibami’s. Aijima turns grief and blame into a carefully anticipated chain of someone else’s ordinary actions.
What the three couples were really doing
The apparent infidelity is resolved as a misunderstanding. Kobayashi, Naeko and Yumi have been planning to take their partners to a Night Hearts concert. The men’s separate meetings concern surprises or advice connected with their actual partners rather than new romantic attachments.
The real pairings therefore remain Chiba with Naeko, Shiratori with Kobayashi, and Shukichi with Yumi. The different restaurant combinations were misleading evidence because the children supplied a romantic purpose before learning why the adults were together.
The story ends by returning the couples to the concert plan. The band carries the bitterness of Aijima’s motive in one part of the case and the possibility of a happy shared evening in another. The latter softens the close without changing the seriousness of the poisoning.
The two investigations share a useful question
Both strands begin with an apparently obvious explanation. People sitting together must be on dates; someone collapsing after dessert must have been given poisoned dessert. Each becomes clearer once the observers ask what happened before the visible moment.
That parallel gives the busy restaurant setting its shape. Haibara’s delighted curiosity and Yumi’s forceful presentation keep the ensemble lively, while the chair mechanism ensures that the murder remains a complete case with its own evidence and motive.
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