The Doctor and The Sexy Romana go to a planet where metal is very scarce and everything is made of wood, which is ruled by Queen Elizabeth the First because she owns the only metal mine. She has The Doctor tossed into a pit with a giant green blob-alien that has a very large penis, who turns out to be an ambassador from a planet with lots of metal that needs some wood in exchange.
The only memorable moments are the casting of the Sexy Romana to replace the Unsexy Romana and the moment when the Doctor meets the giant green blob-alien’s very large penis and attempts to make contact by giving him a blow job, I shit you not. This is widely acknowledged as one of the show’s most embarrassing moments ever.
Re-reading the above description, it becomes apparent just how obsessed this episode is with penises. It's said to have been one of the Russell: The Davies favourites.