Doctor, how very nice to see you again. Seems like only 474 years since we last met.

Probably one of the Fourth Doctor's best stories, it serves as both a cerebral exercise and a very fun romp. Most people probably know this as a Douglas Adams vehice but this one also has the presence of the very underrated writer David Fisher within it (if only because Fisher wrote the kernel called A Gamble with Time that eventually grew into this serial). All of the actors in this are also quite good.