Another adventure for everyone's favourite time traveller. The vast majority of the population labour in an underground city, crippled by the taxes imposed on them by the autocratic Company. It isn't long before the Doctor is siding with the underground rebel movement, but the need to overthrow the corrupt rulers takes on a new urgency when Leela is captured and sentenced to death by their leader, the sinister Collector...

In 1714 Parliament offer a £20,000 prize for anyone who can provide an accurate means of measuring longitude at sea. John Harrison (Michael Gambon) flies in the face of popular opinion by saying that the stars do not provide the answer, and provides his own solution with the invention of a mechanical clock. However, it takes Harrison forty years to prove his theory, and he is eventually forgotten in the mists of time. Centuries later, Robert Gould (Jeremy Irons) attempts to restore Harrison's reputation by tracking down and repairing the four clocks he originally constructed.