A Handful of Stardust

The TARDIS is diverted to England in 1572, and the Sixth Doctor and Peri meet John Dee — "mathematician, astrologer, alchemist, magician, and the greatest mind of our time". ("Only of your time?", the Doctor asks, unimpressed.) But what brought them here? When the Doctor discovers that Dee and his assistant have come across a "great disturbance in the cosmos, in the constellation of Cassiopeia", he realises that they are all in terrible danger.