15. The Space Museum

In the words of Steven Moffat, "Wibbley wobbley, timey whiney". For the first time, we get an episode about time travel! The Doctor and company "jump the time track" and lands in their own future. It's a fun plot device for a few episodes while they figure out what exactly is going on.

But then THE watershed moment of the series finally arrives. The Doctor incites revolution! While the Doctor has encouraged civil war and fighting between parties before, it was usually between a pre-existing conflict. This time, no prior conflict existed. The "ruling" party wasn't out to destroy anyone (see The Daleks), and wasn't really hurting anyone. Yet Vicki finds a group of youngsters to help revolt against their elders, breaks into their weapons locker, and starts a revolution.

This does seem to be about the kids fighting against the establishment. So while this is the first time we're seeing this, it certainly won't be the last in the shows 50 year history.

en.wikipedia.org/wiki/The_Space_Museum

Next heart breaking episode: The Chase!