Seven of Gary (TOS S2E26: Assignment Earth)
When the crew of The Enterprise find themselves back in the 60s, everything seems like smooth time-travel-sailing. But when Gary meets our crew, this simple mission becomes a paradox with an extra cat! Will our fearless crew save that day? Is the future compromised? Where can I get Spock's hats? It's the episode that appreciates your time, and its own time.