The Doctor discovers who the puppet Master pulling the strings, Omega.
The Three Doctors was released as an anniversary special comma and this episode does not disappoint, we are given plenty of information surrounding Time Lord history, and Omega's involvement in the Time Lords becoming so powerful. Omega is a fascinating character, and looks great, Stephen Thorne is very charismatic, and has a vocal skill.
Lots of info that would be used in later episodes has its origins here, Remembrance of The Daleks, Arc of Infinity and spinoff Downtime. Plenty of rich content, I'd love to see Omega return in the new series.
Plenty of fun Dialogue between Doctor number two and The Brigadier, The Brig's stoic character and Pat's jovial character contrast so nicely. It's a good episode for Jo. Sadly we don't get much Hartnell, his health by this stage wasn't great, wonderful to see him though.
Best of the special so far 10/10