- “Blinky: Beware, changelings are swapped with their human counterparts at BIRTH. So, it is likely that these troll-pretenders of dwelt amongst you for decades. They could be ANYONE; used cars salesmen, tax collectors, television executives...
Blinky: Yes! Especially dentists!”
- ―Blinky to Jim and Toby after giving them a Gaggletack to expose changelings hiding in Arcadia
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.
In the backyard, Toby throws a watermelon at Jim so he can practice. Jim slices it cleanly in half then celebrates with a taco. Toby next throws a stack of magazines, which Jim shreds. Just then, Draal comes outside and calls what they're doing a mockery. He says the Sword of Daylight is not a toy. It's an obligation he's spent his whole life training to be worthy of. Jim offers him a turn with it. He takes the sword, but it quickly vanishes and ends up back in Jim's hand. Draal says if his destiny is not to hold the sword, perhaps it is to teach Jim how to wield it. He grabs a nearby lamp and uses it as a sword to attack Jim. He tells Jim the sword is an extension of his body and his body an extension of his eyes. He tells Jim to mimic his movements. He practice into the night. Draal tells him to learn to expect the unexpected and learn to embody the armor, forcing it to do what he wants. Jim uses his powers to get the sword in his other hand and gets Draal off him. Toby's phone chimes, reminding them they're supposed to meet Blinky at the museum. They set off on their bikes.
Jim crawls in through a window Blinky left open for them, followed by Toby. Toby jokes about their prior arrest as Jim notes that the goblins are all gone.
Vendel, AAARRRGGHH!!!, and Blinky are waiting when Jim and Toby arrive. Toby revealed that Draal has been training Jim. Blinky says that arrangement could only end in disaster. Vendel asks if they're planning to show him the thing that might be the Killahead Bridge.
Jim leads them to the Bridge as he tells the story of how he saw it the first time. Toby and Jim work together, but can't pull off the cover, so AAARRRGGHH!!! steps in to do it, revealing not a bridge, but a ship. Jim says Toby took a picture, but Toby says there was no flash and it's out of focus, so maybe they only thought they saw it. Vendel is upset that he came out for nothing. He and Blinky argue over Blinky's insistence on seeing conspiracies. Vendel asks to be taken home and he and AAARRRGGHH!!! leave. Blinky thinks maybe Vendel was right. Jim insists that he saw it and Blinky says the proof must be somewhere. He tells Jim to worry about his studies and he'll worry about finding it.
As Toby and Jim arrive at school, they talk about how they might find the Bridge. Jim's sure they won't be able to stumble on it again, but the curator definitely didn't move it by herself. Toby wonders if other changelings helped her move it. At a distance, they see Claire get out of a car. Jim remembers that when he thought he was going to die, he wrote Claire a letter, too, and told her everything. Claire calls out to Jim, saying she's been looking for him. Claire asks if he hasn't run into Miss Janeth yet. He asks if she's mad he couldn't make rehearsal yesterday. Something came up, he says, as he remembers the battle. But that's what understudies are for, so Steve filled in. Claire tells him Steve's not the understudy anymore. Jim is. She made Steve Romeo. Miss Janeth is on the warpath for him, so he needs to watch out. Jim remembers that he has Algebra with her, too. The bell rings and Jim rushes so he won't be late to Algebra.
Jim sneaks in as Miss Janeth stands at the board trying to explain something. She notices him anyway and orders him to sit in the front row. She talks about the equation, relating it to the play. Then she asks Jim how he'd solve the equation.
Claire finds Jim after class and notices he survived. But now he has a mountain of homework. Claire says she did Honors Algebra last year and was pretty good. She invites him to come over for a study session. He's surprised she'd do that since he ruined the play. She says making Steve Romeo wasn't his idea and she'd rather be on stage with him even if he's a flake. They agree to meet at six.
Jim is lost in thought until AAARRRGGHH!!! shouts and startles him out of it. Blinky is thinking of a plan to figure out what happened with the bridge. They know the curator wasn't working alone. If they can prove that, Vendel might start believing that he really saw the bridge. Jim says they just have to find more Changelings, but Blinky tells him not to say that word too loud because the mere word could cause hysteria. They go to RotGut's to find one. Blinky knocks on the door and an eye peeks through a hole to ask if they want an ailment or a curse. Then another voice says he was supposed to be the one to answer the door. They argue over it while Blinky and Jim wait. Gut peeks back out and asks how they can help. When they say they suspect they might have a Changeling, Rot opens his window back up to say they need a gaggletack. Gut says they're short on those, but Rot says he has a full bag of them. Gut tells Rot secretly that he's trying to drive up the price. While they argue, Jim asks Blinky and AAARRRGGHH!!! about them and learns they're a rare artifact and an object of great mystery. With contact, they reveal the true nature of a Changeling. Rot asks them to step back. Then a horseshoe-shaped piece of metal flies out and hits Toby in the face. Blinky warns them that the Changelings have lived among them for decades and they could be anyone. Toby has a few ideas on where to start.
Toby lowers himself down the rope in gym class and presses the gaggletack to Coach Lawrence, but gets no result. Then the coach orders him to do 50 pushups.
Next, Jim presses it to Eli Pepperjack and also gets no result.
Toby has set up a horseshoe toss. After some time, he's gotten bored because there are no takers.
Steve lisps his way through his lines until he gets hit by the gaggletack. Jim quickly grabs it before Steve can figure out who threw it.
Toby goes around tapping random people with it.
Toby and Jim meet up. Neither of them has found a Changeling. They see Strickler nearby. Neither of them has tried him yet. Jim doubts he's really a Changeling. Toby goes to check, but Jim stops him. Toby agrees to back off, but then snatches the gaggletack and throws it near Strickler's feet. He asks Strickler to pick it up, but Strickler declines, saying he doesn't want to waste idle hands such as Toby's. Jim makes an excuse about their backs hurting, but the Claire comes and picks it up. Strickler walks away after telling Jim to take care of his back. Claire asks if they're still on for studying. Toby is ready to expose a Changeling.
Toby is at the orthodontist and Dr. Muelas is examining his teeth. Toby slips the gaggletack out of his pocket and presses it to Dr. Muelas. He's surprised when nothing happens. Gladys Groe then comes and takes it with a cloth, saying it's not very hygienic.
Jim and Claire arrive at her house and she goes to make a snack. He talks to her brother, Enrique, who is bounding in a swing. He uses the amulet to make Enrique like him, which impresses Claire, who has brought back chips and guacamole. Jim compliments the guac and they share a laugh.
Dr. Muelas has finished with Toby and leaves to go home. Toby realizes he's number than usual. He can't even feel his hands. She says that's because she didn't inject him with novocaine. It was a paralyzer. She noticed he had a gaggletack and had to keep him from running away somehow. Toby realizes that his dental hygienist is a changeling and tries to get up, but falls to the floor. Gladys says they need to get him back into his seat because they have a lot of work to do.
Claire and Jim watch Enrique on the baby monitor as she goes over the math with him. He thanks her for the help and tells her he feels really bad for what's happening with the play. He knows it means a lot to her. She was super mad, but now she knows he has a lot more going on than people think. She then pulls out the letter and says you can't write a letter like that and not expect a conversation. She asks him about the monsters he's battling. He pretends it was metaphorical. She says he can talk to her if he ever needs someone to talk to about "the monsters" and it can be their secret. Jim says that'd be great, then shifts his attention back to math.
Gladys has Toby's mouth braced open and picks out a tool. She says Nomura warned her about them and asks who else knows about the bridge, which she knows was the reason why he's there with the gaggletack. Suddenly, Blinky drops down from the roof. AAARRRGGHH!!! is still up on the roof and can't reach Gladys inside. Gladys tries to attack Blinky, but he manages to push her back with a dentist light. She grabs for her weapon and touches the gaggletack instead, revealing her true form. She throws the gaggletack, hitting some tanks of laughing gas, which they all inhale. Toby pulls his phone out of his pocket.
Jim doesn't hear his phone as Claire is suggesting to him that he talk to Miss Janeth about his situation, saying maybe she'll re-instate him as Romeo if she knows. Claire's phone rings and it's Toby. Jim gets on the phone. Jim hears him laughing and rush out, thanking Claire for the guac.
Jim arrives at the office surprised to see them all laughing. Jim fights Gladys and uses the technique he practiced with Draal to transfer the sword to his other hand and finish her off. They don't have proof now that she's gone. Blinky says he finished the fight. Vendel may still have doubts, but they have their lives. They followed Toby as a precaution, thinking either of them could be a target. They were also curious because oral hygiene is a foreign concept to them. The gas has run out, so they prepare to leave. Toby's concerned about leaving the mess, but Blinky just says Toby can handle it.
The goblins pass pieces as Strickler informs Bular that Gladys has been slain. Bular says her job was done as she'd already brought them the stones from Madagascar. Strickler tells him he should care for those he employs, but he only cares to find the stones so his father can escape the prison. Once they have them all, he'll crush Jim on his own. A stone comes through a portal, informing them that another changeling has been chosen, someone that pleases Strickler.
- 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.