"The Bot-Fighter" is the twentieth episode of Big Hero 6: The Series. It premiered on September 29, 2018.
Big Hero 6 find out about heists with battlebots happening in San Fransokyo, so Hiro, Fred and Baymax were night-patrolling to investigate about this. They hear screams coming from Joe's Diner, so the trio rush to the diner and see a small bot terrorizing people. Due to the bot's size, Fred dismisses it as threat, but the battlebot proves to be stronger than it looked when it began attacking the heroes. Hiro quickly comes up with a plan, and uses his Baymax's rocket punch and his own magnetic abilities to destroy the bot by trapping it in a boiling pot.
Though it seemed they had prevented the bot from stealing, Big Hero 6 later watch Bluff Dunder showing footage in the news about the theft, seeing that another bot had stolen the cash register while the heroes were distracted with the Cerberus Bot. Hiro then notices in the video that Cerberus Bot had a familiar print on its foot, hinting that the bot belonged to Hiro's former bot-fighter associate Stan. Hiro, Go Go and Baymax go to Stan's pawn shop to inquire about this, but they arrive only to see he is already being arrested for his bot's crime even though he claimed he was not responsible for it.
As Hiro saw that other bot-fighters were also being framed and arrested for similar crimes, he realized that many of the suspects would be unable to cause these heists, thinking that Yama could be the only person behind it. Hiro also began remembering his days as bot-fighter, so he used this as an excuse to attend a bot-fight alone because he was famous in the underground bot-fight scene, though his friends disapproved of this so Hiro claimed he would only do it to find the real criminal.
He then went to Yam's office, where a bot-fight was taking place, and he admired Trina while she had a match against Gus. Hiro immediately has a crush on her, and soon she helped him hide from Yama, asking why was he sneaking into Yama's office after he had been blacklisted from the fight by Yama himself. Intrigued that she knows about him, Hiro asks if both had met before, so Trina tells Hiro that he was a bot fight legend and every bot-fighter knew him until he quit and "became a loser", suggesting him to leave before Yama sees him, as his presence could make Yama shut the bot fights down.
Hiro then told her he was making a comeback to the fights and later helps Trina upgrade her bot. Next day, Hiro tells his friends he needed to go once more because he couldn't fight anything last night. He watches Trina defeat Saw Bot with his upgrades and both compliment each other. Trina then shows Hiro her controller, inducing him to become a bot-fighter again. Next day, Hiro started thinking about this and went out with Baymax and Honey Lemon to stop Gus' bot when they saw it stealing money from ATM's.
The team examined the bot and realized someone must've hacked and reprogrammed the bot somehow, assuming it was Yama who did it. Hiro then used this as an excuse to join the competition, and that night he returned with Megabot to enter as a contestant, although the ringleader whistled for two goons to kick Hiro out of the building telling him how Yama had forbid it. Hiro suggested that a rematch between him and Yama would draw a big crowd and it would be good for business, so the ringleader decided to allow him as long as he stayed out of trouble.
The ringleader told him that his first match would be against Trina, but before the fight between them can begin, the police busted into the place and everyone else escaped right away except for Hiro, who was arrested again for a little while until Aunt Cass bailed him out of prison a second time. Frustrated, Aunt Cass confiscated Megabot and grounded Hiro. He then goes to the Ito Ishioka Robotics Lab to tell his friends he was close to find out the truth and only needed one more night and then he'd be out forever. His friends agreed and told him to actually be careful this time, so Hiro went back to Lucky Cat Café to wait until Cass fell asleep to sneak out.
While waiting, he thought about what Tadashi would think, so Baymax shows Hiro a video where Tadashi talks about Hiro wasting his time on bot-fights, but he was a good kid. Hiro then goes out and takes Megabot from Mochi's paws, then contacted his friends and came up with a plan to fight in the competition until he faced Yama so that they would be able to search for evidence in his office. Hiro then had a rematch against Trina and defeated Trina's bot despite its upgrades, which made Trina believe Hiro allowed himself to be arrested just to upgrade Megabot. Still, she claimed to be impressed and kissed Hiro's cheek, which made him blush and then Trina wished him good luck with the rest of his bracket.
Hiro next faced other bots until it was his turn to fight Yama, who had a reconstructed Little Yama. Hiro texted his friends to go to Yama's office while he defeated Yama, using almost the same method he had done first time. Trina watched the fight and gave Hiro a thumbs up after he won. However, his friends searched Yama's office but did not find anything incriminating him, only finding a video of Yama in Samba classes which was too gross for them.
Only then did Hiro realize that the only other person who could've caused the thefts was Trina as she had fought the same bots as Yama. Hiro went to her to talk, revealing that he knew she was the thief and trying to convince her to leave bot fights by mentioning that his brother Tadashi helped him and got him into SFIT, claiming it can turn her life around too. Trina however refused so Big Hero 6 entered the place with the intention to arrest her. Trina then activated all of her hacked bots, including Megabot, to form a larger bot which attacked Hiro and his friends so she could escape. During the fight, Hiro managed to reprogram Megabot and have it defeat Trina's bot by disassembling its parts. After destroying the robot, the team went back to Lucky Cat Café, where Hiro told them he is done with bot fights and he mentioned how he could have ended up like Trina if it hadn't been for Tadashi and Aunt Cass, also thanking his friends for worrying about him.
Elsewhere, Trina returns to her "father" with Kraken Bot. They show him a diamond which they stole but Obake tells Trina that collecting trinkets was a grand waste of time, instead asking her to show him something useful. Trina then opens up her face which reveals her true robotic appearance, and she projects a hologram of Hiro talking about Tadashi to her when he confronted her earlier. Seeing this, Obake told her that "we offered him freedom, and he chose to leash himself again", then proceeded to shut down Trina, and tells her it is time to move forward.
- Trina (debut)
- Mr. Yama
- Yama's Thugs
- Bot Fight Ringleader (credited as Yama's Assistant)
- Little Yama
- Cerberus Bot (debut)
- Kraken Bot (debut)
- Saw Bot (debut)
- Worm Bot (debut)
- Smash Bot (debut)
- Hiro puts his hood up when he hides under a desk with Trina, but while both speak Hiro's hood is back down for one shot and then he's wearing it again.
- After Hiro finds out that Trina is the thief, Hiro is holding Megabot, but during their conversation Megabot disappears. Megabot also disappears when Go Go says "That's it, we're taking her in."