The 602nd episode of Doctor Who airs on PBS television stations across America – the first Doctor Who story ever to premiere outside the UK – with British fans having to wait two days later to see it on BBC1. Patrick Troughton and Jon Pertwee reprise their roles as the second and third Doctors, with Richard Hurndall stepping into the role of the first Doctor. (Never-before-seen footage from the abandoned 1979 story Shada constitutes Tom Baker’s appearance as the fourth Doctor.) Numerous former companions of the various Doctors put in guest appearances as well, alongside Anthony Ainley as the Master. The special 90-minute story celebrates 20 years of Doctor Who.