Finn doesn't mind being the victim of the occasional practical joke. He's pulled more than enough of them in his time, so understands the need for balance. Which is why, when Monty's head goes missing and in its place is a ransom note demanding tickets to Marshall's Park and a personal tour of the zoo, he finds it as funny as the next man—until he realizes it's not a joke, and he may miss Kaylee's show and tell because of it.
Being Sherlock Holmes was not how he'd planned to spend his one weekend off a month, so why is he hiding in the bushes waiting for the kidnappers to collect their reward? Of course, he's certain the culprit is Chris, which is why he plans to make him pay in ways he hasn't even thought of yet. But when he jumps out, he's not sure who's surprised more, him or the kidnapper—who is definitely not Chris.