Stardate 3113.2: On Earth in 1969, the U.S. Air Force mobilizes when they spot an unidentified flying object. They mobilize to intercept the suspected UFO which is… the Enterprise.

The Enterprise was en route to Starbase 9 when it was dragged toward a black hole and the breakaway sent them plunging through space. They pick up a transmission of the first manned moonshot and quickly realize they’ve been thrown back in time. Captain John Christopher is moving to intercept in his fighter and Spock warns he might be able to inflict damage in their damaged state. Kirk orders a tractor beam and the aircraft breaks up. They beam Christopher aboard and Kirk greets the astonished pilot, then brings him to the bridge while explaining that they’re from the future. Christopher is astonished at the presence of an alien, Spock, who warns Kirk that they cannot return him to Earth due to his growing knowledge of future technology.

Kirk meets with Christopher and Spock later in his quarters, where they’re having problems with an overly amorous computer system. Kirk informs Christopher that they can’t send him back and his disappearance won’t affect the future. Scotty lets them know they can re-energize the warp engines, but they have nowhere to go, and Christopher note they are just as trapped as he is.

Later Spock calls a meeting with Christopher but Kirk realizes that he’s making a break for it. The captain intercepts Christopher at the transporter room and knocks him out. Later Spock reveals they must return Christopher to Earth because his not-yet-born son will lead a space probe.

Later Kirk and Spock meet to determine a way to his explain his return. Christopher got picture of the Enterprise and the Air Force has recovered the wreckage. Spock has a tentative plan to return to their own time, and Kirk determines to recover the pictures so they can eliminate evidence of their presence and discredit any report Christopher might make. Christopher provides a map of the base where the pictures are stored and Kirk and Sulu beam down. A security guard catches them and takes their equipment, while Spock calls down for a status report. The guard triggers the emergency signal and they beam him up. They quickly disarmed the shocked guard and keep him isolated in the transporter room.

Sulu and Kirk start to recover the files but set off a silent alarm. Kirk goes to check the files and is captured by a new party, and attacks them as a distraction so Sulu can call for beam out. The commanding officer, Colonel Fellini, demands to know where Sulu is but Kirk denies he had an accomplice and no one could have escaped.

Fellini interrogates Kirk without much luck, while Christopher offers to provide the necessary coordinates… if they take him with them. Spock takes Sulu and Christopher with him and they quickly overwhelm the guards and rescue Kirk. However, Christopher grabs a firearm from a guard and threatens them to make his escape. Spock has circled around and knocks out Christopher before returning to the ship.

Spock and Scotty work out a slingshot maneuver that will propel them first backwards and then into the future. They will beam Christopher back into his ship with no memory of what has happened but Scotty warns they may not have enough control to arrive at the point in time they need to arrive at. Kirk agrees and they break orbit and head for the sun. They execute the maneuver and breakaway, then accelerate to immeasurable speeds. Christopher thanks them for the chance to see the future and then they beam first Christopher and then the guard back to their respective periods with no memory of what happened. The Enterprise makes it to its own time and brakes successfully, where they make contact with Starfleet Command.