Fog has come to San Fransokyo, and Hiro, Baymax and Fred are out on night patrol. However, being so foggy it was hard to notice any trouble. Just then, Hiro and Baymax spot three operatives flying near the harbor wearing jet turbines. The trio call themselves the Mad Jacks: a team of extreme operatives who strive for action. Alistair Krei is out on his yacht and the Mad Jacks attack him. Hiro and Baymax intervene, but the Mad Jacks force them into the water. However, they emerge and successfully fight off the trio, forcing them to retreat. Even though they saved him, Krei does not tell Hiro why the Mad Jacks were after him. He had just bought a computer chip on the black market, containing unknown secrets that were worth stealing.
The next day, Hiro comes down with a fever, probably after getting drenched when he and Baymax were forced into the cold bay. Both Baymax and a visit to the doctor's advise him to stay in bed and drink plenty of fluids to get better. Despite being sick, Hiro stubbornly refuses to rest while the Mad Jacks are out to get Krei and that chip. One night, Hiro receives word from police broadcast that Krei has been kidnapped by the Mad Jacks. Instead of informing his friends and staying in bed, Hiro goes after the Mad Jacks alone on Tadashi's scooter. Hiro fails to save Krei, and breaks his leg in the process.
Because his leg is fractured in two places, Hiro has no choice but to stay in bed and get better. He still, however, remains reluctant to do so, especially when the Mad Jacks have Krei in their clutches. However, the Mad Jacks let him go after he gives up the chip. After the gang receives word from Krei, Big Hero 6, on the account of Hiro confined to bed, start a city wide search for the Mad Jacks. The Mad Jacks arrive at a wearhouse, and present the chip to their cilent, Obake. When they realise Big Hero 6 is on the move, the Mad Jacks recommend keeping them occupied while Obake escapes with the chip. Obake, however, suggests they hold onto the chip until Big Hero 6 is neutralised. The Mad Jacks engage Big Hero 6. Pretty soon, Hiro, having built a life-size remote control robot, joins in the fight from the comfort of his bed. With Hiro's help, they get the chip back and the Mad Jacks retreat.
Despite the Mad Jacks' failure, Obake is not surprisingly disappointed. At Krei Tech, Krei is having second thoughts about handing the chip over to his security team to examine it. Afraid there might be someone on the inside, he decides to inspect the chip himself only to discover that it is blank of any kind of information. Little does he know that the chip is actually a bug that allows Obake to hack into Krei's computers. Once inside Krei Tech's systems, he not only gains access to top secret projects, but also uncovers Hiro's secret identity.