On the day of the classroom observation at Teitan Elementary School, a fire breaks out in the flowerbed in front of the classroom. The newly transferred student Danno Kaisuke claims this is the work of the vengeful spirit of a "man-eating classroom". However, Conan sees through that the fire was set with an automatic device: a burning lens made of ice, which focused sunlight onto potato chips serving as an accelerant, ignited the fire at a predetermined time.

Conan identifies the culprits as his classmates Haibara Ai and Higashio Maria. Maria wanted to cancel the classroom observation to spare her injured mother, and asked Haibara for help. Haibara built the device and melted the evidence, the ice lens, after the fire was extinguished using warm 10-yen coins in her pocket to eliminate the traces. Conan catches on to them through damp pencil leads and their lack of small change.

In parallel, Conan's parents, Yusaku and Yukiko, talk about the undercover investigating assistant teacher Wakasa Rumi. They confirm that she is the former bodyguard of a US politician and has infiltrated the school to get closer to the organization through Conan, to which she has sworn revenge. Rum from the organization also shows himself informed about her activities.

The episode ends with Maria's mother attending the class via video conference and the two culprits being sentenced to replanting the flowerbed.