The reptilian Sea Devils are very closely related to the Silurians; but whereas the Silurians dwell on land, the Sea Devils adapted to underwater life. Like the Silurians, they inhabit colonies containing thousands of their people in hibernation, and their colonies are hidden around the world. And like the Silurians, they wish to destroy man and reclaim the Earth for themselves.
The Sea Devils arm themselves with metallic disc-like handguns which flash a concentrated beam of heat when fired. They can use this not only to kill mammals, but also to cut through rock and metal. They keep the handguns tucked into a pouch at the waist of the net tunics they like to wear. Tall, graceful and quick even when out of the water, the Sea Devils have turtle-like faces with swept-back fins on the sides and large fishy eyes. They are bipedal, and they seem to require oxygen to breathe, although it could be that they simply prefer breathing oxygen to breathing water. When they speak, their voices are like rasping whispers. They touch palms as a gesture of friendship. And there is an indication that they may possess an empathic sense, or at the very least, they are good at reading humans.
Technologically, the Sea Devils are apparently able to stop submarines dead in the water. Their weapons are capable of destroying ships. They have a special kind of diving bell which they can use to transport prisoners to their bases. Their harbors are equipped with forcefields which keep enemy ships from escaping (and which perhaps keep other things from entering?). Sonar also seems to be highly important to the Sea Devils. They have a device apparently based on sonar which can track vessels in the water, their hibernation devices are triggered with sonar, and their communications equipment apparently uses ultra-high-frequency sonic beams. Sonar can also be used to incapacitate them with head pain.
The first time they encounter the Doctor, one of their colonies at the base of an abandoned sea fort is awoken when the foundation of the fort is rebuilt and it is converted into a sonar testing station. Soon three ships go missing, and a lifeboat is found burned and sunk. The Master is able to construct a signal and control device to summon them, at which point he offers to repair their faulty hibernation unit so that they can take over the Earth. As a result of thousands of years of disuse, their reactivation machinery is in need of a new trigger mechanism, and the Master believes he can use a laser to substitute for the usual sonic device, and thus revive all the Sea Devils from their hibernation. The Doctor encourages them to try for peace with the humans, but a poorly-timed attack from the Navy convinces the Sea Devils that they must take the offensive, so they storm the naval base. Although they are forced to retreat, the Master manages to finish building his device and fulfill his part of the bargain, at which point the Sea Devils promptly discard him. But the Doctor, having foreseen their refusal to make peace with man, has reluctantly insured the Sea Devils' destruction by reversing the polarity of the neutron flow on their trigger device. Upon activation, it explodes hugely, destroying the Sea Devils' base completely.
Their next encounter with the Doctor comes after a chamber of Sea Devil warriors has been revived by a group of Silurians. These Sea Devils seem to be subservient to the Silurians, and they also wear a different costume: an armored tunic, a skirt and a helmet. They are sent on board the Sea Base as an advance party, and use a cutting device to get through the doors. They succeed in overrunning the base, but in the end they are annihilated by hexachromite gas, which seems to affect Sea Devils more quickly and severely than it affects Silurians.
