The Mystery he needed to solve was that there was a man who came from a really rich family was missing. Everyone thought he got kidnapped by someone. Sean suspected that it was the sister, because he thought the sister had killed him so she could get all the glory from his father. But actually it was the father who killed him, but the father didn't mean to kill him, he accidentally killed him while he was getting mad at him, and threw his sons head to a corner of a table. The father didn't want the cops to know it was him, so he called the cops trying to make the cops think it was someone else, rather then the guy trying to fake being a victim.
But during that time, the dog bit the father so then Sean figured everything out, and started revealing to the cops how the father still had that mark from the dog. And eventually the father made an admission that he "Accidentally" did it, and got arrested
