Krei first appears at the San Fransokyo Institute of Technology convention, where he was conversing with a young inventor about his invention before noticing Hiro's Microbots. He quickly takes interest and attempts to have Hiro sell him the Microbots, but Callaghan intervenes, saying Krei is known to cut corners and subtly manipulates Hiro into refusing Krei's offers. He leaves angry and disappointed. Later, while Hiro was tracking Yokai in the abandoned warehouse and after surviving a car chase from Yokai, he shows his friends a symbol of a bird in a circle which he noticed plastered on a wall of the abandoned warehouse where Callaghan had manufactured the microbots. Hiro shows this symbol to the team after they narrowly escaped a car chase with Yokai. This made Fred believe Alistar Krei was really Yokai, though everyone else dismissed this as it would bring Alistar Krei negative attention and be too risky for his public image.
Once Hiro and the team head off to an island off the coast where Baymax tracks Callaghan's location (having scanned Yokai's body) which turns out to be an abandoned Krei lab. As the team explores the lab, they discover an old computer with the same bird symbol Hiro had seen earlier. It then demonstrates a video of Alistar Krei performing a demonstration of a portal that can be used to teleport. He first uses the general's hat as an example, which was a success and next tried the same thing on a human being, who in this case was Abigail. One of his scientists stated there was a malfunction but he dismissed this and had the scinetists procceed. Unfortunately, the experiment failed as the portal began to malfunction and suck everything in with it, Krei was forced to shut his experiment down and abandon his lab under quarantine. This incident would also later earn him the wrath of Callaghan, as it was presumed his daughter Abigail, who was the test subject, had died and he blamed Krei as Krei had proceeded with the experiment despite knowing their was malfunctions, indirectly killing her. This made Hiro and the team believe Krei became Yokai in an attempt to steal back his research. However, when they unmask Yokai, they are shocked to see that he is actually Callaghan. After he escapes and Baymax calmed down Hiro, who burst into anger after learning that Callaghan didn't have remorse for killing Tadashi, the team watches the rest of the clip to get some answers, and realize the test subject was Abigail who was Callaghan's daughter and that Callghan was seeking revenge against Krei for the accident.
Callaghan attacks Krei during his latest ceremony as he reveals his attentions to take away everything precious from Krei after he caused the accident that took away his daughter. Krei begs for mercy, saying it was an accident and that he would give Callaghan anything he wanted. However, this only angers Callaghan as he states that he wants his daughter back. Big Hero 6 intercepts Callaghan and attempt to reason with him, acknowledging his loss and how his daughter would not approve his actions. Callaghan for a moment expresses regret, however Krei's cowardice causes Callaghan to resume his plans and pins Krei to a wall while he battles Big Hero 6. Krei is soon freed from the wall and watches as Hiro and Baymax enter the portal to retrieve Abigail and Hiro comes back with Abigail safe at hand.
Krei is last seen opening a showcase hall in Tadashi's honor in the film's credits, apparently in consideration for the young man's sacrifice.
Hiro and his friends mistake him as the true identity of Yokai until he is revealed as Callaghan. Despite this, he still responsible for the inevitable turn of Callaghan into Yokai, due to his pursuit of science making him forget about the safety of people around him (which the worst accident was the one that thought have killed Abigail).