As incidents thought to be the work of the kamaitachi demon occur one after another, the innkeeper's father-in-law, Bonzo Ohno, is murdered in the annex of the inn. Based on the crime scene, Heiji believes the culprit came to the annex through the outdoor bath. However, the snow around the bath is undisturbed, meaning the culprit could only have run across the bath's surface. After hearing everyone's testimony, Conan and Heiji realize who the culprit is from a casual remark.