Sontarans thrive on combat and delight in the rigors of warfare. They reproduce through cloning programs meaning they can be created in huge numbers - ideal for a race of soldiers literally born to fight. When Sarah Jane Smith makes her first appearance as the Doctor's companion in The Time Warrior, she must help confront a Sontaran in the Middle Ages. Back in the modern-day, David Tennant's Doctor is reunited with Martha Jones for a two-part story where the Sontarans threaten humanity with a deadly poison.