When the Doctor returns to Gallifrey, he learns that his bio data extract has been stolen from the Time Lords' master computer known as the Matrix. The bio data extract is a detailed description of the Doctor's molecular structure — and this information, in the wrong hands, could be exploited with disastrous effect. The Gallifreyan High Council believe that anti-matter will be infiltrated into the universe as a result of the theft. In order to render the information useless, they decide the Doctor must die...