“The neurotransmitter must be in his mask. If we get the mask, he can't control the bots. Game over.”
Yokai's Mask was a mask worn by robotics expert Professor Robert Callaghan while under the supervillain alias Yokai. It was a white mask with red markings and yellow eyes, giving it a sinister Kabuki appearance. It was equipped with a neurotransmitter, giving Yokai the ability to control the Microbots. Without the mask, Yokai would lose control of the Microbots.
During a scuffle between Yokai and the Big Hero 6 team (consisting of Hiro Hamada, Baymax, and Hiro's friends who were Callaghan's pupils) at what used to be a testing facility (owned by Alistair Krei) on Akuma Island, Hiro managed to remove the mask, and Yokai was revealed to be Callaghan. Callaghan demanded Hiro give him back the mask, to which Hiro responded by corrupting Baymax and ordering him to terminate Callaghan. As rest of the team attempted to stop Baymax from committing murder, the professor retrieved the mask and escaped with the Microbots.
In the final battle against Yokai, the Big Hero 6 team initially aimed for the mask, but Callaghan proved to be unstoppable due to his control of the Microbots, so Hiro had to come up with a plan to strip Callaghan of his power. After the Microbots were eliminated, the mask was finally destroyed by Hiro and Baymax, thus causing Callaghan to permanently lose control of the Microbots.