One of the defining characteristics of Doctor Who is that from time to time, a new actor would take over the title role. The practice got its start when the star of the show, William Hartnell, decided to retire. But rather than cancel the show, the producers decided to recast the role. The explanation they came up with was that the Doctor, like all members of his race, the Time Lords, could "regenerate" his body when it gets severely injured or worn out. When I started watching the show in 1980, the Doctor was played by Tom Baker, the fourth actor to play the part.
At that time, information about the Doctor Who was hard to come by. There were a few imported items available in a very few shops, but that was it. There was no shop near me that sold such items, and it's not like I could look up information about the show on the Web, so I knew next to nothing. I'd just turn on the TV every Sunday night at 11 PM and watch whatever episode happened to be on that night.
On one of those nights, my brother happened to be visiting from college. He and my mom were talking in the family room, where I normally watched, so I went to my room to watch. The episode that night was called "Logopolis." The Doctor's arch-enemy, the Master, was up to no good, trying once again to take over the universe. And there was this mysterious figure in white called the Watcher who seemed to be stalking the Doctor. Toward the end of the episode, the Doctor and the Master faced one another over on the catwalk of a giant radio telescope. One thing led to another, and the Doctor ended up falling off the catwalk and plummeting hundreds of feet to the ground.
The Doctor's friends ran to his side. He looked up at them. "Don't worry," he said, "this moment has been prepared for." The Watcher came out of nowhere, walked up to the Doctor, and their bodies sort of merged into one another. The Doctor's face disappeared, as if covered with thick cobwebs. And when the cobwebs cleared, there was another actor there. I had just witnessed a regeneration, one I'd had no idea was coming. It was the most amazing thing I'd ever seen. Twenty-two years later, I've still only seen a handful of things that even came close to it.
So that's why Doctor Who is still such a big deal to me. And it's why I'm generally anti-spoiler. "Logopolis" is a good episode regardless, but it would not have had anywhere near the same impact had I known at the start it was Tom Baker's last episode. The unfortunate thing is that I'm unlikely to ever have that kind of experience again. If Sarah Michelle Gellar decides not to do an eighth season of Buffy, it's going to be impossible to avoid the news. Sure, I won't know how the series will end. But it won't be the same.