Merlin has a plan to go to war, but it's Jim's choice to make. Blinky and AAARRRGGHH!!! search for allies. Gunmar uses Barbara to get what he wants.
To help Jim prepare for the upcoming battle against Gunmar, Morgana, and Angor Rot, Merlin creates a special potion that will transform Jim into a half-troll to strengthen his abilities. However, when he tells him that there will be no going back, Jim has to face whether he should keep half of his humanity or not. Meanwhile, Blinky and AAARRRGGHH!!! search for allies, and Gunmar kidnaps Barbara in order to force Strickler into releasing Morgana.
With everyone debating with Merlin about his goal for killing Morgana, Jim is still in mourning about the destruction of his Vespa. Merlin states that the only way to guarantee the defeat of Gunmar is to release Morgana. As long as he has the Staff of Avalon, her release is indeed imminent. This is when they will strike, before she has a chance to bring forth the Eternal Night. However, everyone is debating about going up against not only a troll army, but a troll warlord, a troll assassin, and a troll-using sorceress. But, if Jim does defeat Morgana once and for all, Merlin gets his magic back as well. The only two people who agrees with the old wizard's reasoning are Claire and AAARRRGGHH!!!, as everyone else think he only cares about getting his magic back. Merlin settles with leaving Jim the choice of going to battle or to war up to him as he is the protector of the world of humans and trolls, but Jim decides he must think. Whatever he decides, Blinky and AAARRRGGHH!!! decide to assemble and army of reinforcements.
Gunmar's forces have captured Strickler and Barbara and dragged them down to the chamber in Trollmarket where Morgana is. Strickler refuses to align himself with a monster like Gunmar, but begins to reconsider when he sees Barbara unscathed. All Gunmar asks is that he unlocks the Heartstone with the staff. Barbara says to do exactly what he says, and nobody in Arcadia will be hurt. But Strickler notes that Barbara always called her "Walter", and reveals that it was really Usurna with a glamour mask. With Angor Rot disappointed with how he knew that wouldn't work, Strickler once again demands to see Barbara.
Back at Jim's house, him, Claire and Toby are trying to reach her by phone, to no avail, and nobody has seen her anywhere. When Toby tries calling anyone at the school if they've seen her, Nomura bursts through the front door, with a dislocated shoulder and bleeding. She tells everyone about her attack at the Museum, saying that Gunmar needs a changeling to activate the Staff of Avalon. Merlin spitefully says that Gunmar found a way around his safeguard: The incantation to activate it must be spoken by a human, and Jim realizes that Gunmar has Strickler since he didn't get Nomura, meaning he also has his mother. Nomura gets ready to fight while Claire and Toby search for her. Merlin halts Jim back, saying that he cannot let him intervene, despite knowing Jim's mother is with Gunmar. He claims that it's best if Strickler activates the staff now so they can end this war with Morgana once and for all. But Jim protests, prioritizing about saving his mother over Merlin's desire for his magic back. He then fires a beam out of the amulet at Jim's phone when he tries texting Toby to meet at the canals for a rescue mission to Trollmarket. Jim's anger over Merlin unwilling to cooperate reaches a boiling point, and he summons his armor. Merlin then summons a broad sword of his own, and they battle each other.
Blinky and AAARRRGGHH!!! continue with assembling the comrades, their first stop being Gatto's Keep. However he's (as usual) unwilling to cooperate.
Back in Trollmarket, the Gumm-Gumms have brought forth Barbara in a cage. Gunmar is prepared to release her and only her so long as Strickler activates the Staff of Avalon, to which he reluctantly agrees (despite Barbara's pleads).
Back at Jim's house, Jim is losing the swordfight to Merlin. Jim recalls that he had a choice, but Merlin corrects, saying that the choice was never about whether to fight, but how to fight. He further claims that Jim's humanity or the Eclipse Blade will not be enough to vanquish Gunmar. Leaving him no choice, Merlin tears the amulet from Jim's armor, rendering him unconscious.
Meanwhile, Blinky and AAARRRGGHH!!! continue their search for reinforcements at the Quagawumps camp, only to find they have reluctantly struck a deal with Usurna and the Gumm-Gumms for protection, much to Blinky and AAARRRGGHH!!!'s chagrin.
Back at Jim's house, Jim awakens from the fight, tied to a chair watching Merlin formulate his spell with the ingredients he sent the gang to find. As he puts the finishing touches on the formula, Merlin points out that he does care about the world and Jim's mother. But he just sees the bigger picture. He then reveals what the ingredients were for: to turn Jim into a troll (half-troll, to be precise). Since Jim couldn't even hold his ground against Merlin, there's no telling what could happen if he even attempted going toe-to-toe with Gunmar, Angor Rot, or even Morgana. The way Merlin's elixir will work is that Jim will submerge himself in water that he pours it in, and he will come out as an entirely different creature. Jim asks why it has to be him. Merlin says he needed Jim's heart, claiming that he once had a heart as pure as Jim's, until it became more jaded throughout his centuries. In order to end this stalemate once and for all, he needed a champion capable of having a foot in both worlds. Jim was chosen to be the first human Trollhunter, but he was never destined to remain a human. The only way to be the hero everyone needs Jim to be is to physically become both troll and hunter. Jim then asks how long he would be like this. Merlin sorrowfully answers that once he uses the concoction, there will be no going back for him and he will remain a half-troll for the rest of his life. In order to save his world and his friends from the forces of evil, he must sacrifice half of his humanity and leave his place in the human world behind. Merlin realizes that this is the greatest call anyone's ever asked of him and respects that it is still his choice to make: to either remain human and risk both of the worlds' survival or forever become a "true" Trollhunter and even the odds.
In Morgana's chamber, Gunmar is ready to proceed. With Angor Rot holding Barbara hostage, Strickler activates the Staff of Avalon. As it shoots a beam meant to break the Heartstone, Strickler redirects it at Gunmar and Angor Rot, opening a window for them to escape. The staff serves its purpose and the stone bursts, revealing the vile sorceress has finally returned, ready to bring forth the Eternal Night.
Back at the house, Jim has made his choice: he is going through with Merlin's plan.
Strickler and Barbara make their escape from Trollmarket, with the pursuing Gumm-Gumms petrifying in the afternoon sun. They meet up with Claire and Toby about Morgana's release and Claire tries to call Jim.
He doesn't answer, however, as he is still thinking. Staring at the amulet and at himself in the mirror, Jim remembers all the good times he's had as a human all the while summoning the armor. Just then, the gang bursts through the door, searching for Jim, horrified to learn the truth about what Merlin was having him do. When they hear that Jim is in the bathroom, Jim hears his mother and friends pounding on the door, begging him not to go through with this. Jim thinks about everything he and those closest to him have gone through: from the day he first found the amulet, his battle with Draal, his refusal of the call, killing Bular, Barbara learning about Trollmarket for the first time, venturing into the Darklands alone, rehearsing for Shakespeare with Claire, and so on. He momentarily pauses when he hears Claire's pleas, but when he learns that Morgana is released, he realizes that the Eternal Night is now inevitable and he must transform himself in order to save both of the worlds. Without saying a word, he sorrowfully climbs into the magical concoction with everyone still knocking on the door. Jim remembers what Blinky said on the first night they met (about destiny being a gift, fear being the precursor to valor, and to strive and triumph in the face of fear is what it means to be a hero).
The gang gets the door open too late; Jim has fully submerged himself in the elixir, entering what appears to be some sort of NetherRealm. As a portal he entered begins to close, Jim sinks down a dark abyss with nothing to hear but his own heart beating. The amulet flashes on his chest and the portal in the bathwater disappears, leaving everyone wondering what happened to Jim, and where he is now.
The credits roll in with nothing to be heard but the ominous hissing of the NetherRealm, and Jim's heart beating and breathing sharply as he begins his transformation.
- Kelsey Grammer as Blinky
- Emile Hirsch as Jim Lake Jr.
- Charlie Saxton as Toby Domzalski
- Lexi Medrano as Claire Nuñez
- Jonathan Hyde as Mr. Strickler
- Fred Tatasciore as AAARRRGGHH!!!
- Ike Amadi as Angor Rot
- David Bradley as Merlin
- Clancy Brown as Gunmar
- Grey Griffin as Queen Wumpa
- Mark Hamill as Dictatious
- Lena Headey as Morgana
- Anjelica Huston as Queen Usurna
- Amy Landecker as Barbara Lake
- Lauren Tom as Nomura
- Laraine Newman as Nana
- Matthew Waterson as Draal
- Morgana is set free in this episode.
- This is the last episode of Trollhunters to feature Jim as a human. After this, he will remain a troll/human hybrid forever.
- The music that is heard at the beginning of the episode is "The Flower Duet" from the opera Lakmé by Leo Delibes.
- While trying to decide on whether or not to give up half of his humanity, Jim has a flashback that recaps the entire series.
- Anton Yelchin is credited in this episode, as he provides the majority of Jim's dialogue during the flashback montage near the end of the episode.
- Jim being asked to give up his place in his world in order to save it is the most mature and selfless sacrifice he has ever made, especially from a sixteen-year-old human boy.
- Getting Crap Past the Radar: The entire sequence of Jim giving up half of his humanity and everyone begging him otherwise at the end of the episode contains massive allusions to suicide.
- To add another layer of horror to this allusion, the sequence has an unsettling visual resemblance to a bathtub suicide––the act of killing oneself by sitting in a tub and slitting one's blood vessels.
- Title: The episode's title is a reference to the House Divided Speech, an address given by later President of the United States, Abraham Lincoln, on June 16, 1858, after he accepted the Illinois Republican Party's nomination as that state's senator. One of the best known lines from this speech is, "A house divided against itself, cannot stand."