The Sontarans are a warlike race of clones from the high-gravity planet Sontar. They inhabit the universe of Doctor Who. They are bipedal humanoid creatures that are slightly shorter than humans. Because of the high gravity of their home world, they are extremely strong. Their skin is dark brown. Their heads are dome-shaped, and slope continuously down to meet their shoulders, leaving them with no real neck. Their hands have two larger fingers and a thumb. They are not very pretty to look at. They are always seen wearing their tight-fitting black body armor. This armor includes a dome-shaped helmet, which they do not always wear outside of battle unless they are trying to masquerade as a human. Being that they are a race of clones, they all appear more or less identical. There are some minor deviations between them, however. On the back of their "neck" is a small aperture -- their probic vent. If it is blocked or struck hard, it can incapacitate or even kill them. This is their main physical weakness. Arrogance is their primary mental weakness.
The Sontarans started off very different from the race they became. They previously used sexual reproduction instead of cloning. The shift to cloning happened when General Sontaris, a powerful military leader, developed technology that allowed him to clone himself. He created millions of such clones, and using this army he took over the government and destroyed all of the non-clones. He renamed the planet and the race after himself. The entire race, from that point forward, were clones of General Sontaris, either directly or indirectly. Some small modifications and variations have been introduced in the many years that have passed since this event took place.
After the Sontaris clones took over, the Sontaran race became extremely warlike and aggressive, conquering every other race they encountered. Eventually they ran across the Rutans, the first race that they were not able to dominate. Equally matched, they did battle for thousands of years, with neither one ever getting the upper hand.
Over the course of the Doctor Who television series, the Sontarans are encountered four times.
The first encounter happens in The Time Warrior (1973-1974, 3rd Doctor). Commander Linx of the 5th Sontaran Battle Group is on a scouting mission when "he" crash lands on medieval Earth, having been shot down by a Rutan squadron. Linx encounters a group of humans, but quickly surmises that they do not possess the technology required to repair his damaged ship. He manages to construct a device that will allow him to abduct scientists from the future. He uses these scientists to repair his ship, and also to provide a human warlord with firearms that will allow him to become king. Unfortunately for Linx, the Doctor discovers this activity, and travels back in time. He and Sarah Jane Smith (in her first appearance on the show) manage to rescue the scientists. In the process, Jinx is killed and his ship is destroyed.
The next encounter with the Sontarans takes place in The Sontaran Experiment (1975, 4th Doctor). In the distant future, the Doctor, Sarah Jane, and Harry Sullivan visit the Earth. They transmat down from the human-run Space Station Nerva, and they find the Earth in a tranquil state, with everything overgrown by plants. The Doctor and friends run across the survivors of a crashed spaceship. They are humans from the colony of Gal-Sec 7, who came to Earth to answer a distress signal. When they arrived, their ship was destroyed. Since then, Field Major Styre of the Sontaran G-3 Military Survey has been conducting a series of tests on the humans. The tests are designed to aid the Sontarans in a planned invasion of the humans. The Strategic Grand Coucil of the Sontarans is waiting for the study to be completed before they commence the invasion. The Doctor, Sarah Jane, and Harry manage to stop the study before it is completed. They destroy Styre and blow up his ship. This was accomplished by exploiting the Sontaran arrogance -- the Doctor challenged Styre to a duel, and while this was taking place, Harry sabotaged Styre's ship. Because Styre's study was not completed, the invasion was called off.
In The Invasion of Time (1978, 4th Doctor), the Sontarans use the Vardans, a race of telepaths, as their tool to get access to the Matrix on Gallifrey. The Matrix is a vast repository of knowledge and memories of the Time Lords. The Doctor tracks down the Vardans, at which point the Sontarans invade Gallifrey. The Sontarans are repelled by the Doctor after occupying Gallifrey for only a short time.
Finally, in The Two Doctors (1985, 2nd and 6th Doctors), the Androgum race and the Sontarans are working together to try and master time travel. They kidnap the 2nd Doctor, but the 6th Doctor, Jamie, and Peri manage to rescue him. The main Sontarans involved are Varl and Group Marshal Stike, who end up being double-crossed and killed by the Androgum. The Androgum are killed before they can escape with the time travel device.
The Sontarans appeared in the following television episodes: