The Crusade

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:
The TARDIS arrives in 12th Century Palestine where a holy war is in progress between the forces of King Richard the Lionheart and the Saracen ruler Saladin. Barbara is abducted in a Saracen ambush and the Doctor, Ian and Vicki make their way to King Richard's palace in the city of Jaffa. Ian is granted permission to ride in search of Barbara - the King knighting him Sir Ian of Jaffa to fit him for the role - while the Doctor and Vicki stay behind and try to avoid getting involved in court politics.

King Richard secretly plans to marry his sister Joanna to Saladin's brother Saphadin in order to bring the war to an end, but Joanna finds out about this and refuses. The Doctor and his young ward are forced to flee after making an enemy of the King's adviser, the Earl of Leicester.

Ian has meanwhile rescued Barbara from the clutches of the vicious Saracen emir El Akir. All four meet up in the wood where the TARDIS materialised and narrowly manage to escape the Earl of Leicester's men.

5 out of 79 (6.3%) raters say this story requires a previous story.