Case Closed - Season 1 Episode 635

Ran, Sonoko, and Conan are at a yoga weight-lost resort when one of its customers, Atsuko Degawa, dies from poison found in her food. Originally deduced to be a suicide, Conan reveals the culprit as the resort's owner, Tsukiko Satoyama, who had poison laced on the plastic wrap which mixed with Degawa's food when it was re-heated in the microwave. After finding the poisoned plastic wrap on her dog, Satoyama confesses and reveals Degawa's negative reviews caused her parent's business to shut down and as a result, had her parents commit suicide.