Thorsuun was a Euterpian. He was part of a five member recon team from the Euterpian mothership that landed on Earth around 40,000 BC. They planned to refuel the mothership from Earth's magnetic core, but before that happened, the mothership was destroyed by solar flares. The recon team (Atimkos, Godwanna, Thorsuun, Udentkista, and Tarwildbaning) were the only survivors. Their plans to destroy Earth in 1994 were defeated by the Second Doctor.