Sea Devils are an intelligent reptilian species that live in underwater, air-filled cities at the bottom of Earth's oceans. The are the underwater cousins of the Silurians, and like them retreated into hibernation chambers many thousands of years ago when a planet threatened to crash into the Earth. Many Sea Devils are still in hibernation awaiting the signal to awake: their instruments have yet to give the all clear because the collision never occurred. Only a few colonies have come to man's attention as a result of his investigations of the seabed triggering the Sea Devils' alarms.

Once rulers of a great underwater civilization, the Sea Devils are not a martial species. Together with the Silurians, they made great scientific advances and developed a sophisticated culture. Those colonies that have woken to find man, a jumped up ape, running and ruining their planet naturally wish to remove this usurper. If the Sea Devils could trust man, however, and man could trust the Sea Devils, it would be possible to share the planet. The third Doctor tried, but failed, to achieve this compromise in The Sea Devils.

Sea Devils are manlike lizards with a tough, green and brown leathery skin. Their turtle-like heads have beaked mouths and golden eyes with large black pupils. On each side of the head, a fan-shaped frill of skin sweeps backwards from the temple to shoulder level. They dress simply in a blue net robe that is tied at the waist by a white gun belt. Sea Devil warriors wear helmets and armour similar to that of medevial samurai. Although Sea Devils are reptilian, they are much like humans in their mental make-up, with the same emotional frailties and strengths.

Sea Devils carry a compact, circular blaster that uses heat energy. Hexachromite gas is a deadly poison to Sea Devils.