The Terileptils are an intelligent, technologically advanced, yet war-like reptilian race. The Terileptils are as dedicated to beauty and art as they were to war, mixing the two with ease. A group of four Terileptil outcasts who escaped from the prison planet of Raaga became stranded on Earth in 1666. These fugitives, realising they were stranded, attempted to destroy humanity with a virulent strain of plague, in order that they might make the Earth their own. Their number had been reduced to three by the time the Doctor became involved The Doctor tracked them down to a bakery in Pudding Lane. The resulting fight ignited a fire -- the Great Fire of London!

The planet Raaga is covered in tincalvic mines. Ragga is a Terileptil penal colony and all prisoners who serve there end up sustaining major disfiguring burns while mining the tinclavic. The Terileptil leader sustained major disfiguring burns there before he escaped to Earth.