Lethe (river)

Lethe was a river on the planet Gallifrey. When the young First Doctor and the Master were playing there, the Master was nearly drowned by a bully named Torvic, who the Doctor then killed. The Master realised that if they were found out, they would never be able to leave Gallifrey. They therefore burnt the body. (AUDIO: Master)

Behind the scenes

 * In Greek mythology, the River Lethe is a river that erases the memories of all who drink from it. In Master, the Doctor forgets killing Torvic after making a deal with Death.