|“||Oh, so you're my replacements - a dandy and a clown.||”|
— The First Doctor being a grumpy old cunt.
It's the first multi-Doctor story and establishes the concept of the Doctor being able to meet past incarnations. It also ends the Third Doctor's interminable exile to Earth. Was also the first time the Brig and Sergeant Benton enter the TARDIS.
Also, something about William Blake.
Unfortunately by this point Hartnell was deteriorating rapidly due to various diseases and could only sit in a chair and read cue cards, so the First Doctor's appearance is confined to showing up on a monitor and yelling at the other two occasionally. Anyway after the usual padding, Two and Three discover Omega's behind it all.
It's a bit dated and filled with bad special effects but it has some great banter especially between Troughton and Pertwee. The Brig is awesome as always.
The whole back story of Omega is fairly interesting, even if the actor playing him seems to think he's doing Shakespeare and is over the top as all fuck. Still, it's worth a watch.