Another all time favorite!
A welcome return by the Master is outshone only by the introduction of the aquatic cousins of the Silurians - the Sea Devils, who like the Silurians have a very impressive costume and voice. They are a welcome addition to the classic Who "monsters" and are, to me, iconic right up there with the best of them.
A great cast, eerily different music, and a huge help from the Royal Navy make this a particularly memorable story. Essential Third Doctor.
(Love it so much, I made it my name!)
SeaDevil