|| Oh my god|
I was wrong
It was Earth
— The Doctor
The Mysterious Planet is earth. Spoilers.
It's also the first episode of Season 23 aka. The Trial of a Time Lord.
It stars Colin Baker as the Valeyard, Michael Jayston as the Doctor, Nicola Bryant as Peregrine Took and introduces Tony Selby as Sabalom Glitz.
The Doctor is summoned to Gallifrey and once again put on trial by the Time Lords for homosexuality. Thus begins the trial of the afore-mentioned Time Lord (BRAVO SAWARD).