"Seventh Wheel" is the twenty-seventh episode of Big Hero 6: The Series. It premiered on May 7, 2019 and is the second episode in the second season.
Liv Amara is having a private meal served by Momakase at her hideout, as she has once again escaped from prison. As she dines to her perfectly made dinner, Amara asks for Momakase's services in obtaining an "investment" for her company, and in return, she promises to pay her handsomely. Momakase accepts her offer.
At Frederickson Manor, Fred is clearing out Mini-Max's rocket thrusters after using them to make s'mores, when suddenly a masked ninja swiftly breaks into the kitchen, takes down Fred and Mini-Max and escapes. Fred later shows his friends a recording of the break-in taken from the "microwave camera". At the same time, they are still trying to get used to Professor Granville knowing they are Big Hero 6. She even starts butting in, trying to help them. According to Fred, the thief stole nothing valuable from his home, not even his genuine underpants from a Captain Fancy movie. At first, it all seems a dead end until Professor Granville points out that the case the thief was carrying is temperature-controlled. Whatever the thief stole, had to be stored at precisely 1.6 Degrees Celsius, which is the recommended temperature for consumer refrigeration. By following this line of reasoning, the thief stole something from Fred's fridge. They go back to the scene of the crime and find that Mr. and Mrs. Frederickson's valuable White Stilton Gold cheese is missing from their private fridge. They also receive a tip from Professor Granville on where the thief will strike next. Splitting up, Hiro, Baymax, and Fred head for the Caviar Bazaar in time to see the thief steal White Beluga Caviar. The turns out to be Momakase and makes her escape with the caviar. Hiro and Baymax ambush her on the rooftops and engage her in a fight. Baymax switches to overdrive mode and almost beats Momakase until his battery runs low. This gives Momakase the chance to take him down with his own sword and escape.
Baymax has suffered severe damage. Many of his systems are fried, including his language settings, which are causing him to speak different languages. While affecting repairs, the team wonders why they never received any alerts about Momakase getting out of jail. However, they again receive another tip from Professor Granville. According her friend Gary, who is the prison's warden, Momakase escaped and the police wanted to keep it quiet to avoid alarming the public. Fred then shows up with chicken noodle burgers for everyone, only for Granville to insist they eat nutritious food, like kale nuggets. She recommends that they go on night patrol while Hiro stays behind to fix Baymax.
At Sycorax, Amara is working on Momakase. After scanning her bio-metrics, she creates a bio-chip and injects it into Momakase's neck. By morning, Hiro has spent the whole night fixing Baymax and is almost finished a few more problems to solve. He fixed his language settings but his scanner is still on the fritz. Plus he now has a malfunction that randomly inflates his body parts. Fred, Go Go, Honey Lemon, and Wasabi arrive back from night patrol, feeling very tired. All five of them fall fast asleep until Granville wakes them and gives them a high-intensive workout. Afterward, the team agrees that Granville is going overboard with helping them and decide to have someone tell her to tone it down. They suggest Hiro, but he does not wish to repeat the mistakes he made last semester. The last time he did not listen to Granville his actions almost destroyed the city.
Later, Granville and Big Hero 6 start formulating a plan to catch Momakase. Fred mentions that he has to attend some fancy dinner party where the chef picks a color and only uses expensive food with that color. The team realizes Momakase was only stealing white food and that the party is going to take place at her old hideout. Instead of splitting up as they originally intended, Granville recommends Big Hero 6 to go in together, catching her off guard. The team surrounds Momakase in her kitchen and engage in a fight. Momakase lures them all into a pantry and traps them inside, which is impenetrable against Wasabi's Plasma Blades and Fred's Fire Breath. Hiro notifies Granville of what has happened, which makes her realize her mistakes. So she decides to go and set things right, starting with fixing Baymax.
Back at the cooking arena, Momakase serves up dinner and pays Amara. In return, Amara proceeds to activate the chip she injected into Momakase. Granville and Baymax arrive at the restaurant and engage Momakase in a fight. Granville realizes she is outmatched, so she decides to use a different approach: Running. Baymax finds the team and uses overdrive mode to break them out. Meanwhile back at the arena, Momakase slices open a bottle of cooking oil which spills on the floor, causing Granville to slip and fall. Right before Momakase can kill Granville, Hiro uses his magnets to take her sword. Momakase then reveals what the chip did to her, causing her fingernails to grow and transform into Graphene. The fight continues with Momakase shredding everything the team throws at her. So they combined their powers to overwhelm her, eventually trapping her in ice. Granville apologizes for her intrusiveness, and they accept. Suddenly, Momakase escapes her icy prison, now looking like some weird monster with grey skin, mane-like hair and eyebrows, thick linings around her eyes, and sharp canine teeth. She escapes the arena and runs off into the night.
- The title is a reference to "third wheel", an idiom that refers to an additional person being unnecessary or gratuitous to the current party (usually of two people). In this case, the title alludes to the team having six members, therefore a seventh member would be excessive.
- When Momakase prepares food to Di Amara at the beginning of this episode references Food Fight when Momakase served Alistair Krei food.
- During training, Wasabi wears the same outfit as he does in his short with Baymax.
- Hiro still has Obake's CPU that he found after the Kentucky Kaiju was electrocuted and destroyed at the end of Kentucky Kaiju.