Missing episode

Arriving in Central Asia in 1289, the Doctor and his companions join the caravan of the famous Venetian explorer Marco Polo as it makes its way from the snowy heights of the Pamir Plateau, across the treacherous Gobi Desert and through the heart of imperial Cathay.
 

Having witnessed many incredible sights and survived a variety of dangers, they arrive at the mighty Kublai Khan's Summer Palace in Shang-tu, where the Doctor strikes up an extraordinary friendship with the now-aged ruler.

They move on at last to the even more sumptuous Imperial Palace in Peking, where the travellers save the Khan from an assassination attempt by the Mongol warlord Tegana — supposedly on a peace mission — before departing once more in the TARDIS.

The TARDIS arrives on the planet Marinus on an island of glass surrounded by a sea of acid. The travellers are forced by the elderly Arbitan to retrieve four of the five operating keys to a machine called the Conscience of Marinus, of which he is the keeper. These have been hidden in different locations around the planet to prevent them falling into the hands of the evil Yartek and his Voord warriors, who plan to seize the machine and use its originally benevolent mind-influencing power for their own sinister purposes.