Catherine Tate (Donna), Freema Agyeman (Martha), a Sontaran, and David Tennant (The Doctor) in a publicity still for season 4's return of the Sontarans.
"I am delighted to be back filming on Doctor Who with David and Catherine," said Agyeman, who had swapped over to Who spin-off Torchwood for that show's second season. "Martha Jones returns in a fantastic storyline and the fans will get to see her in a whole new light!"
The Sontarans are a cloned warrior race in perpetual conflict with their arch-enemies the Rutans. The Earth seems to be in a strategic position in their conflict, and Sontarans have visited Earth periodically to experiment on humans. They are short (in most episiodes), wear heavy armor, and have no necks. Sontarans will duel for their honor if they are insulted, but otherwise consider other, weaker races beneath them.
The Sontarans originally appeared in 1973 in the Third Doctor episode "The Time Warrior," a fan favorite not only for the introduction of a fascinating new race, but for the debut of the well-liked companion Sarah Jane Smith (Elisabeth Sladen) as well as the sharp byplay of a sparkling script by Robert Holmes. The potato-headed warriors later successfully invaded the Doctor's home planet of Gallifrey (The Fourth Doctor episode "The Invasion of Time") and were last seen again plotting to control time travel in 1985's Sixth/Second Doctor story "The Two Doctors."