Amphibious relatives of the Silurians, the Sea Devils are also a prehistoric race which went into hibernation millions of years ago in reaction to their discovery of the Earth and Moon on collision course. Luckily this catastrophe didn't occur but the hibernation persisted. Like the Silurians they are determined to reclaim Earth from the Human interlopers.

The Third Doctor first encountered the Sea Devils as they allied with the renegade Time Lord, the Master, in The Sea Devils, after he awakes a colony under the English Channel. They once again surfaced during the Fifth Doctor's time in Warriors of the Deep, alongside their land-based Silurian cousins.