After the Hamadas return home following another incident of Hiro's bot-fighting activities, Cass starts stress-eating donuts after lecturing the boys on their irresponsibility and goes to the kitchen, calling to a sleeping Mochi to come with her, to which the cat complies.
Later, when Hiro and Baymax return home after trying to investige his Microbots' activity, Hiro tries to get Baymax upstairs without his aunt noticing. Mochi appears, purring and rubbing against hiro, prompting Hiro to claim the sounds Baymax is making to be from Mochi. When Cass turns away again, Hiro throws the cat upstairs. When Hiro arrives in his bedroom, Mochi is seen being petted by Baymax and purring in his arms while calling him a "hairy baby". Mochi notices Hiro and leaves the room, still bitter about Hiro throwing him. When Hiro and Baymax sneak out again, Mochi is shown with Cass watching Frankenstein.
Mochi stars in the short "Baymax and Mochi", where Cass leaves Baymax to take care of him while she's out, but the cat makes a mess all over Lucky Cat Café, then escapes after Baymax attempts to put him in the dishwasher for a bath. The robot follows him all over town and in the end both return to the Café. At night, Cass finds Baymax in his charging dock with Mochi sleeping on his arms.
In the episode, Mochi learns how to clap slowly with his paws, and Honey Lemon records him then uploads the video to internet. The video becomes a hit due to its cuteness and Mochi becomes famous. However, the video unadvertedly causes Mr. Sparkles' popular internet challenge show to lose fans, which makes him angry and jealous of Mochi. Mr. Sparkles finally snaps when people put him aside to take photos of Mochi, so he goes and captures the cat while Cass is being interviewed, places him in a rocket and announces that he'll launch it into the sun unless someone completes one of his challenges. Hiro tasks Baymax to go with Cass and prevent her from finding this out. Both go around San Fransokyo pasting "Missing" flyers while Hiro builds a robot to save him and the rest of Big Hero 6 try to get the challenge done. Go Go succeeds but Sparkles had gone mad enough to launch him regardless, right before escaping. Fortunately, Hiro arrives in time and sends his new robot, Skymax, to bring back Mochi. After Mochi is saved, his reunion with Cass is recorded and posted to the website MeowTube where the video becomes another hit as well.
- Mochi - Mochi is a Japanese rice cake that is filled with a flavored paste. It is a popular treat, and can be filled with bean paste or ice cream. Mochi is usually made in a special mochitsuki ceremony, and is usually served around the Japanese New Year.