Case Closed - Season 1 Episode 718

A mystery writer calls on Mouri Kogoro to investigate if there's anything going on between her husband and her assistant. Kogoro, Ran, and Conan stay in a different house overnight, and come morning, they find out all three people dead from poisoning. The husband dies first after eating poisoned sleeping pills. The assistant dies next after drinking poisoned tea. The writer dies last due to poisoned chocolate bars. Conan finds it odd that there is spilled water and two unopened chocolate bars in the garbage in the writer's room. Finally, it is deduced that the writer initially planned to commit suicide by eating poisoned chocolate bars and stage it as a murder done by the husband and assistant, but after seeing that they are both dead, dies herself by eating the same chocolate bars by mistake.