I missed that episode. Could you elaborate?
To sum up:
Rassilon was a peer of Omega. The pair were involved in creating the means by which time travel was possible. As a consequence, Omega was trapped in an antimatter universe at the bottom of the black hole that powers the TARDISs. And for whatever reason Rassilon was entombed, alive, inside what was later called the "death zone." During the Five Doctors, the Lord President of Gallifrey summons the first five incarnations of the Doctor (though #4 and Romana #2 get stuck in the time vortex) to free Rassilon so he can have immortality. The others, along with various companions (retiring Brigadier Leftbridge-Stewart, Sarah-Jane, the current companions of #5) are deposited in the zone, face cybermen, daleks, and other threats. And the Master is also captured to help - and is offered a new regeneration cycle for it.
Interesting that the Master would be summoned to free Rassilon.
Sidenote: The episode also features what appears to be Zoe and Jamie, post trial, but who remember their travels with the Doctor - a fact that allows #2 to figure out they are imposters. Except that this #2 was visiting the Brigadier at his retirement in 1988 (also seen during the McCoy era), and thus should not remember events that have not yet happened for him. (Immediately after their memory wipe and return home, he was forced to regenerate into #3).
For some reason, that others here have mentioned, during the time of McGann, the Council released Rassilon, and impeached President Romana. This is why he is President during the Time War.
btw, did anyone else have trouble with that link. I need to know if the problem is on my end or not. I was having similar problems a week ago, and reinstalled Flash. If it is on my end, that was the first sign.