The Enterprise is thrown back in time to 1960s Earth.
When the Enterprise is flung back in time while trying to escape the gravitational pull of a black star, they find themselves in orbit around a 1960's Earth. When they are seen by a U.S. Air Force pilot, they beam him aboard but then face the dilemma of what to do with him as he learns more and more about the future. They have to review their initial decision to just keep him when historical records show that his yet-to-be-born son will lead Earth's first successful mission to probe Saturn. Spock devises a plan to do so while also erasing any memory of recent events.
- Uh Oh. Big Mistake. The Enterprise just arrived in our century. They have to get back. But the US Air Force saw them, and sent two fast planes to see what they are.
One of the pilots is Captain Christopher. He is a good man with a wife. But his ship is too close to the Enterprise, and he is about to explode, until Kirk beams him on board. Big Mistake. Now Captain Christopher knows everything about the future, and he could destroy the world. But Kirk can't kill him to silence him.
Spock says to Kirk "You can't let him go back." But then the pilot, Captain Christopher, says "I have a wife." Spock looks in the History book, and he tells Kirk that Captain Christopher did absolutely nothing important in his whole life, so they CAN keep him, because taking him would not change History.
Kirk is happy about that, because he didn't want to kill him. But LOOK OUT, Spock looks at the rest of the book, and guess what? Captain Christopher's future son (who hasn't been conceived yet) changed the world like you would not believe. His son will be the leader of the first expedition to Saturn.
So now they can't keep him OR kill him, they have to put him back. Also, by the way, the Air Force took pictures of the Enterprise, so first things first, Kirk needs those pictures back. But guess what? They don't know the way around the Air Force library on the base.
Captain Christopher says "I know it, I will show you." Kirk says ok, but he is worried that Captain Christopher will try to escape, then Goodbye World. Captain Christopher has a plan to run. They get the pictures, and stop him from running away. We TOLD you we would try to get you back, so stop running, Captain Christopher.
Now it is very tricky, but they finally get Christopher transported back inside his plane at the exact moment that he thought he saw the Enterprise. Wow, perfect timing. Now the Enterprise has to use a special technique to get themselves back home. It is scary and several yeomen fall down during the turbulence, but they get back home safe and sound. Wow.