The episode began with a nerdy college kid (Russell Holmes) in Milwaukee being asked for spare change from someone he thought was homeless. Having no money, he offered him his banana instead, and then was captured by the Unsub.
When three partially buried bodies with bite marks are found in a Milwaukee park, the BAU was called to investigate.
Turns out they all died from untreated rabies infections. The Unsub was an animal control expert, whose brother had contracted rabies from a bat at the age of nine. By the time he was diagnosed, it was too late to administer a vaccine. His parents took him home and then euthanized him rather than see him suffer.
The brother's death triggered a psychopathic tendency in the Unsub (David Cunningham). He later pursued a career with animal control, specializing in dangerous and trapped animals.
Cunningham captured his first victim, subjected him to a bite from a rabid animal, and then used that victim to facilitate a bite attack on his next victim. All three victims later died of rabies.
When the BAU were called in, the Unsub had at least two victims in cages: the first, fully rabid, was a 42 year old stay at home mom (Liz Foley), and the second was Holmes. Cunningham used Foley to bite-attack Holmes, infecting him with rabies. Foley later escaped and began walking around town threatening people.
The BAU surrounded Foley in a park and captured only after Morgan shot her in the leg. She was taken to hospital where she later went into a coma, brought on by the rabies.
The team eventually located Cunningham's lair. In the process of capturing him, Cunningham tazed Morgan and Reid tackled him. They got Holmes to the hospital where he received rabies treatment in time.
The episode ended with Reid and Garcia discovering that Morgan was to be their PT instructor for a mandatory fitness test. After he subjected them to several strenuous exercises, he told them that he had already waived their test, and was just having fun at their expense.