After stopping Globby from stealing a piece of Krei Tech, Hiro learns from Krei that it was a broken piece from a failed Energy Amplifier. Inspired, Hiro decides to focus on recreating it for open house finals, only to be turned down by a visibly distraught Granville. Nonetheless, he continues with it, becoming so preoccupied that he ingores his friends. Honey tries to do a project on Lenore Shimamoto, so she and Wasabi break into her secret lab and find an old wax filter.
Fred concocts a plan to capture Globby by presenting a fake amplifier, but he arrives with Noodle Burger Boy who unknowingly tampers with Baymax due to Hiro arriving late. At the open house, Honey plays Shimamoto's message and she reveals herself to be the cause of the "great disaster" in the past. Obake reveals himself to Granville and has Noodle Burger Boy, Globby and Momakase attack the open house so he can steal Hiro's amplifier. Afterwards, Granville (who is aware of Big Hero 6's identities) explains how she knows Obake.
The tampered Baymax kidnaps Hiro and takes him to Obake who reveals his plan to create a star to destroy San Fransokyo so that he can rebuild it with Hiro as his progeny. Globby has a change of heart and frees Hiro and they unite with the rest to stop the impending destruction. Honey and Globby brace the flood while Go Go and Wasabi protect the civilians and fight Momakase. Hiro, Fred and Mini-Max then use Kentucky Kaiju to create a powerful wave that disrupts the star and causes Obake's lair to crumble. Obake frees Baymax and sends him to rescue Hiro while he resigns himself to his fate. Granville decides to pass Hiro for his heroic deeds and he admits that he is unsure if Obake survived.
- Ryan Potter as Hiro Hamada
- Scott Adsit as Baymax, Guy in Green Shirt
- Génesis Rodríguez as Honey Lemon, Female Scientist
- Khary Payton as Wasabi
- Jamie Chung as Go Go Tomago
- Brooks Wheelan as Fred
- Jenifer Lewis as Professor Granville, Female Student
- Andrew Scott as Obake
- Lucas Neff as Noodle Burger Boy
- Andy Richter as Globby
- Naoko Mori as Momakase
- John Michael Higgins as Mini-Max
- Alan Tudyk as Alistair Krei, Guy in Hat
- Jeff Bennett as Baron Von Steamer
- Haley Tju as Karmi
- Mara Wilson as Liv Amara
- Riki Lindhome as Wendy Wower
- Christy Carlson Romano as Trina
- Diedrich Bader as Bluff Dunder
- Maya Rudolph as Aunt Cass, Instructional Voice
- Jeanne Sakata as Lenore Shimamoto
- Obake’s real name is revealed to be Bob Aken.
- Obake’s plan to destroy the city of San Fransokyo is explained.
- He also explained that he intends to make Hiro his student after the city’s destruction.
- The secrets of Lenore Shimamoto’s research have also been revealed.
- When Globby attacks the open house as a dinosaur, a banner floats down in front of him briefly. This shot recreates the famous climax scene from Universal's Jurassic Park.
- Hiro's cover he uses for Obake when talking to Cass is that he was out collecting "Obakes" on his phone. This is clearly a reference to the popular phone app Pokémon Go.
- It is revealed that Granville has known about the real identities of Big Hero 6.
- Granville's security passcode is 4-4-7-6, which reads as "HIRO" on a standard telephone pad.
- Krei is still upset over Globby stealing his wallet in "Big Roommates 2".
- Wendy Wower is now referred to as "Doctor Wower".