This was an early episode of The Doctor Who series and one which has been lost. In the early days of the Doctor Who, once a series had been shown, it was then discarded. It wasn't just limited to Doctor Who series as the policy also affected other serials as well such as Dad's Army. The series features William Hartnell as The Doctor.
The Tardis arrives in the Himalayas in 1289 and the travellers happen to chance upon Marco Polo, the famed Venetian explorer who would travel between Europe and China on the silk road. The Doctor and his companions, Ian Chesterton, Barbara Wright and Susan Foreman work to prevent the assassination of Marco by Tegana, a warlord.
In addition to ensuring the safety of Marco Polo, they also need to get the Tardis back as Marco Polo had taken it in order to get back on good terms with the Chinese Emperor.