"Lie Detector" is the thirty-fifth episode of Big Hero 6: The Series. It premiered on May 17, 2019 and is the tenth episode in the second season.
Big Hero 6 patrol the streets of San Fransokyo searching for an elusive Kentucky Kaiju-like monster. Go Go gets the drop on the monster, who is revealed to be Fred in a different suit, as part of the team's training exercise to prepare them for future monster attacks. In a secret lab at Sycorax, Liv Amara is getting nowhere with treatment on the mysterious figure. Chris comes in and offers her a golden-flaked cupcake, giving Amara the idea of using gold for biocompatibility.
Back at Frederickson Mansion, Wasabi tests the team on two-truths and one lie. No one except Baymax deduces the third statement to be a lie, as his health scan picked up on Wasabi's eyes dilating and his heart rate increasing. When Fred compares Baymax to a lie-detector, Hiro sets out to upgrade Baymax's lie-detecting abilities with the intent of finally exposing Amara. After Hiro installs Baymax with a new lie-detecting chip, the two follow Amara downtown to a restaurant where she is meeting Bluff Dunder. Hiro and Baymax listen from a table nearby. Amara tells Dunder she is working on a project that requires gold, but before Hiro can hear more, he is informed by Go Go via text that Mr. Sparkles has returned.
Sparkles and the Mayoi try to rob a freighter. Once Hiro, Baymax, and Go Go appear, Mr. Sparkles has the Mayoi equip him with scuba gear and dives underwater. The heroes catch up with the aid of Skymax-delivered underwater suits (including a new one for Go Go). Sparkles calls upon the fully-mutated High Voltage for backup. The team handles High Voltage and Go Go traps the villains in a whirlpool. Sparkles then has the Mayoi dump crates into the ocean, allowing the villains to escape, albeit without the crates they came to steal, which contained gold, cementing the villains were working for Amara. Amara, notified of Big Hero 6 interfering with her plans, decides to unveil her secret weapon: Bessie.
Meanwhile, Baymax's lie-detecting causes trouble for Hiro and the others in covering up lies. Shortly after, Amara assigns Chris on a mission before she leaves for a meeting. Hiro and Baymax show up wanting to ask Amara, but Chris stops them. Hiro asks Amara if she is the one creating the monsters and Amara denies, stating "Liv Amara" is not creating the monsters. After she leaves, Baymax "dings", indicating Amara was telling the truth to Hiro's bewilderment. At night, Chris retrieves some fur from a bear in the woods. Amara scans the bear DNA from the fur and fuses Bessie with it, resulting in a new creature. At Lucky Cat Café, Aunt Cass serves Hiro a distasteful acai-salami recipe. Hiro lies about the food tasting good and makes an excuse to leave. Baymax asks Hiro why he lied to Cass about liking her dish, to which Hiro explains he wanted to spare Aunt Cass's feelings. Baymax takes this to means lying is good, but Hiro clarifies lying, in general, is bad, but in certain situations, when you need to help someone, it is necessary.
The next day, a monster attacks San Fransokyo Trust. After equipping themselves their Ultra Armors, Big Hero 6 travels to confront Amara's newest creation—a bear-like rock monster infused with Bessie's energy. As they do battle, Bessie releases electromagnetic beams that disable the team's tech one by one. Baymax receives a blast and seemingly shuts down, but he then sucker-punches the creature, as he lied about failing to save Hiro. Angered, Bessie irradiates a massive energy burst and gets away with the gold.
Hiro removes the lie detector chip from Baymax and resolves to stop Amara but wonders why she is behind the monsters. At Sycorax, Amara addresses the subject that she got the gold, telling them she would do whatever it takes. In the final shot, the comatose subject is revealed to be the real Liv Amara, and the "Liv" who is tending to her is an imposter.
- It is revealed in this episode that "Liv Amara" is not who she appears to be, and the real Liv Amara (who first appears as a silhouette at the end of "Something Fluffy") is in suspended animation.
- This episode reveals that Wasabi started a competitive ironing team in high-school, collects vintage typewriters, and knitted his first sweater when he was 6.
- It is revealed that Fred owns 59497 comics.