To Catch a Changeling is the seventh episode of Trollhunters.
Armed with an ancient troll-hunting tool, Jim and Toby set out to unmask the Changelings living among them. Claire makes Jim a tempting offer.
After Jim finds the Killahead Bridge has been moved, he must use a gaggletack (a horseshoe-shaped artifact that reveals the true nature of a changeling by physical contact) to find who in Arcadia is a changeling. If he touches a changeling with the gaggletack, they change from human form to troll form, so Jim and Toby go around the school touching everyone with the gaggletack to see who is a changeling.
- Kelsey Grammer as Blinky
- Anton Yelchin as Jim Lake Jr.
- Charlie Saxton as Toby Domzalski
- Lexi Medrano as Claire Nuñez
- Ron Perlman as Bular
- Jonathan Hyde as Mr. Strickler
- Fred Tatasciore as AAARRRGGHH!!!
- Brook Chalmers and Tom Kenny as Rot and Gut
- Guillermo Del Toro as Dr. Muelas
- Laraine Newman as Miss Janeth
- Melanie Paxson as Gladys
- Cole Sand as Eli Pepperjack
- Matthew Waterson as Draal
- Victor Raider-Wexler as Vendel
- Thomas F. Wilson as Coach Lawrence
- Steven Yeun as Steve Palchuk
- The song that is heard in the scene of Dr. Muelas while singing is "Cielito Lindo".
- Title: The title most likely refers to the TV series "To Catch A Predator".
- The music heard during the series finale end credits can be heard during Jim and Claire opening up to one another just before Claire reveals the letter Jim wrote her in the previous episode.
- Getting Crap Past the Radar: When Claire reveals Jim's letter in front of him, she mentions that Jim "has to battle monsters", and asks what kind of "monsters" Jim battles. Claire further asks if Jim is under any sort of trouble, and reassures him that if he should need to talk someone about "the monsters", she's willing to listen. It's fairly clear that Claire thought Jim might have major depression, if not suicidal tendencies.
- Running Gag/Continuity: A troll attempts to eavesdrop on Blinky discussing secretive trollhunter information, with Blinky reacting with "Do you mind?" and the troll responding nonchalantly with "Whatever." This is shown in the previous episode, as well.
- Error: When Toby calls Jim regarding his dental hygienist being a Changeling, Jim's phone is seen on his right side. When Jim is seen hurriedly collecting his belongings to leave the Nuñez Estate, he is not seen taking his phone.