Doctor Who: The Key to Many Worlds (excerpt)

The Doctor and Constance arrive in thirteenth century Baghdad and encounter an ageing and disenchanted Marco Polo. His adventures at an end, he has been ordered to transport a certain enchantress back to the Doge’s palace in Venice. The Doctor realises his danger too late, as his mind is swapped with that of the enchantress - the notorious Iris Wildthyme!

Now clapped in irons and trapped in Iris's form, the Doctor is dragged along in Marco's caravan as it trails across the desert once more... and only Mrs Clarke can save him.