The Enterprise is flung back to the twentieth century and must retrieve an Air Force pilot without altering history.
The pull of a black star throws the Enterprise back to the late 1960s, where the US Air Force spots it in Earth's atmosphere. A jet is sent to intercept, and when the ship's tractor beam damages the plane, Kirk beams its pilot, Captain John Christopher, aboard.
Spock learns that Christopher's future son will play an important part in history, so the pilot must be returned without what he has learned. Kirk and Sulu beam into an Air Force base to retrieve evidence, and Kirk is briefly captured. The ship then uses a slingshot around the Sun to travel forward in time, returning Christopher to the moment before he saw it.