Cobra and Mongoose is the first half of the fifty-fifth episode of Big Hero 6: The Series. It premiered on October 26, 2020 and is the first segment of the fifth episode of Season 3 alongside "Better Off Fred".
Hiro and Baymax play an online fight match at Lucky Cat Café against Fred, who is at his own house, when suddenly a robotic cobra appears out of nowhere and attacks Fred, spitting venom that paralyzes him. Heathcliff attacks and destroys the robot snake, and soon Hiro and Baymax go to Fred's house because they overheard what happened. Hiro examines the snake while Heathcliff arrives to give Fred his "lunch", then quickly leaves—but when Fred sees that the lunch plate is actually a cell phone, Heathcliff calls to explain that the cobra was actually after him and because of it, he must leave the Fredericksons as otherwise he will be putting them all in danger, then asks Fred not to follow him. Despite his request, Hiro, Baymax, Mini-Max and Fred use Basemax to track Heathcliff down, revealing that he was currently at Caviar Bazaar. Fred tells Hiro that Mini-Max and himself would go alone and without their suits just to find out what is going on with him.
At Caviar Bazaar, Fred and Mini-Max find Heathcliff but are quickly attacked by another robot cobra, and then a mysterious woman who Heathcliff recognizes as Cobra appears to continue sending the robot snakes at them. Heathcliff destroys the snakes with his umbrella-sword, so Cobra decides to make an escape in her car—while Fred is again paralyzed by another cobra. Heathcliff takes Fred back to Big Hero 6's base, where Hiro watches the security footage and notices that Cobra stole his neurotransmitter tech to use for controlling the robo-cobras. Heathcliff then reveals to Hiro and Fred that before becoming the Frederickson's butler, he was a spy who fought against Cobra on multiple occasions and earned the nickname Mongoose; until he had to team up with Boss Awesome to thwart Cobra's ultimate scheme—and after Heathcliff saved Boss Awesome's life while making Cobra fall down a volcano, Boss Awesome welcomed him into the Frederickson family to remain undercover from the spy life. Still—Heathcliff was determined to defeat Cobra on a final one-on-one showdown, so he asked Fred again not to follow him as he prepared to leave the mansion. Fred initially agreed, but after remembering how Heathcliff had been there for him his whole life and was more than just their butler, decided to go help him.
On a rooftop, Heathcliff and Cobra prepared to face off when Cobra activates her neurotransmitter—even though she had agreed to fight Heathcliff without any tech. She tells him that while climbing up the volcano, she realized the reason she always lost to him was because she always fought alone, but now that she controlled the robotic snakes she wasn't alone anymore. At that moment Fred and Mini-Max join in, and Fred tells Heathcliff that they would fight together because he is an important part of his family. Yet, several robot snakes get into his suit and Mini-Max tries fighting them off, so neither of them are actually able to assist Heathcliff. Cobra gets ready to attack Heathcliff, but he transforms his black suit into a white one that is immune to the cobras' venom, allowing Heathcliff to easily destroy them. It is then that Cobra reveals a gigantic cobra which she uses to continue fighting, and was close to defeating Heathcliff until Hiro and Baymax arrive and take the neurotransmitter from her, thus disabling all the robot snakes while Hiro tells her to get her own tech.
Heathcliff captures Cobra while Hiro and Baymax call the police to have her arrested. Meanwhile, Fred is paralyzed once more, but says that he is proud that Heathcliff is part of his family. Heathcliff thanks Fred, and tells him he will take him to the nearest drive-thru to buy nachos while Baymax and Hiro take Cobra into custody.
- Cobra (debut)
- The most probable inspiration behind this episode is the story of Rikki-Tikki-Tavi from The Jungle Book; in which an Indian grey mongoose is adopted by a family, and where the antagonists are cobras who threaten said family, but the mongoose kills them. The episode parallels this with Heathcliff (Mongoose) being taken in by Mr. Frederickson years before, and Cobra threatening the Frederickson family, then facing off against Heathcliff.
- Real mongooses are immune to cobra venom, which may also be an inspiration for Heathcliff's nickname given to him after defeating Cobra in the past—as well as his white suit seen in the episode which is impervious to Cobra's paralyzing attacks.
- This is the second episode where Go Go, Honey Lemon and Wasabi are absent, which makes Hiro and Baymax the only main characters to be present in every episode of the series.
- Heathcliff floating down with his umbrella during the battle with the cobras is a reference to Mary Poppins.
- Boss Awesome is shown in a flashback but has no speaking lines, most likely due to Stan Lee's passing.
- Running gag: Fred is paralyzed by the cobras' venom. Nachos are mentioned throughout the entire episode.
- The volcano where Heathcliff and Boss Awesome fought Cobra, Mount Krakatuvius, is named after two volcanoes which both had devastating eruptions in the past: Krakatoa in Indonesia—which had a massive eruption in 1883,and Mount Vesuvius in Italy—which destroyed Pompeii and several other cities in 79 AD.