The U.S.S. Enterprise and her crew are diverted by a signal from the former captain of the U.S.S. Enterprise on Starbase 11, and proceed to the call. When they arrive they find Captain Christopher Pike, who is severely crippled by radiation burns, confined to a moving chair and his ability to communicate limited to the answers "yes" and "no."
Unbeknownst to Captain Kirk, Spock abducts Pike on board the ship. Captain Kirk returns to the U.S.S. Enterprise with Commodore Mendez, and soon discovers that Spock has locked the ship's controls on a course set for the planet Talos IV — a planet to which a visit carries the death penalty. Forced by the extreme actions of the Vulcan first officer, the two convene a court-martial against Spock.
During the proceedings, from an unknown source, they watch the events that transpired when Captain Pike was in command of the U.S.S. Enterprise. They are shown Pike's initial contact with the Talosians, a race of beings eager to study human beings in their natural state, and who can provide illusions to make things appear exactly as they would like. The transmission relays Captain Pike's entire mission surrounding the Talosians and their bizarre experiments while holding him in a powerful cage. The Talosians are telepathic beings, able to dig deep into the memories to find whatever illusion will be most effective. They attempt to hold Pike captive until his passion and fury force them to release him.
As the proceedings end, the entire court-martial is found to be meaningless, a result of Commodore Mendez being an illusion created by the Talosians for Kirk's benefit. The events, as well, which depicted Pike on Talos IV were also found to originate from the Talosians.
Because of Pike's condition, Spock has risked his career and freedom to bring his former captain to a place where he will be able to live an illusion of a pleasant life, rather than the tragically limited one he lives now. Kirk allows Pike to beam down to Talos IV and all charges against Spock are dropped.