The seventh mystery is Footprints in the Snow.
It starts by introducing us to a love triangle between Matthias de Gorne, his wife, and his neighbor Jerome. That night, three gunshots are head from his house. Matthias completely disappears, and he is presumed dead.
It snowed last night, starting at 9 PM. The snow sets a firm timeline for when things happen, and the tracks in the snow show what happened. According to the footprints, Matthias de Gorne came home drunk. His house was disturbed, as if a large fight occurred. Someone dragged something heavy to the well, and Jerome left for his house, with the wife in tow.
The police catch Jerome and the wife, trying to flee the city via train. Jerome says that he and Matthias were both at the home, around 6. They made a business deal; Jerome more or less bribed Matthias with a lot of money, in exchange for leaving the country. Matthias signed the paperwork, and started a physical altercation, which explains the disturbed house. Jerome says he tied Matthias up and left with the wife.
There are a few discrepancies between Jerome's version of events and what the footprints show. Jerome says Matthias was there, several hours earlier. He didn't hear the gunshots, and he has no explanation for why something heavy, presumably the victim's corpse, was dragged to the well.
Our detective hero, Arsene Lupin, realizes that this is an elaborate frame job. Jerome was telling the truth about what happened. Matthias framed him, by dragging something heavy to the well, to make it seem like a corpse had been hidden. He then walked backwards through the snow, to make it seem like he had entered after 9. He had his father corroborate his alibi and used "he was super drunk" as a cover for why the footprints looked weird. A close examination of the footprints shows that the heel is way too deep for someone walking forward.
This is the only story from the book I remembered, years later. I forgot all the details, besides for the solution. It's a good mystery story, although I think Matthias was more of a jerk than he should have been, and the way both men treated the wife was kind of awful, in my opinion.