This was a four-part serial that was first broadcast between 14th January - 4th February 1967. The serial was written by Geoffrey Orme, produced by Innes Lloyd and directed by Julia Smith. The script editor was Gerry Davis.
What I've seen:
Episode three. The other three episodes are missing. [I understand that episode 2 has resurfaced, but I haven't seen it.]
The Doctor and his companions find themselves in Atlantis, where mad scientist Zaroff has promised the locals that he will raise their city to the surface. What the Atlanteans don't know is that Zaroff is deceiving them: in fact, he wants to go down in history as the maddest of all mad scientists by destroying the world. There's a lot of other stuff going on - humans being surgically altered so that they can breathe under water and supply food for the Atlanteans, human prisoners who are being forced to work in the Atlanteans' mines, an impending food shortage because the plankton that is the Atlanteans' main source of nourishment goes bad after one or two days - but you may as well forget about all that, since none of it seems vital to the plot.
This is one serial that might actually benefit from there only being one episode left. Some good bits:
The not so good bits:
Twentyfive pretty entertaining minutes.