#60: Assignment Earth
In case you forgot: The Enterprise's latest mission is to travel through time to watch late 60s Earth for "historical study." They stumble across a previously unknown alien being whose ultimate mission is to apparently babysit Earth and keep us from doing anything stupid.
Roddenberry wrote this as a backdoor pilot (get your minds out of the gutter) for his latest brainchild, which would center around said alien being (left), his cat-wife (not pictured) (we also had a woman as a cat in Cat's Paw, what is it with TOS and female felines?), and his plucky young attractive sidekick (right).
This is backassing to the highest degree. Watching the pilot for another show when you're trying to enjoy your Trek is a little tedious, even inconsistent: the previous two times the Enterprise traveled through time it was by accident (The Naked Time and Tomorrow is Yesterday, in case you wanted me to cite my sources) (though I haven't gone back and checked the stardates on the episodes), now they can do it at will? And the last time they ended up back in late 1960s Earth, all they cared about was getting out without messing up the timeline. That seems to be the last thing on anyone's mind in Assignment: Earth.