This story is a sequel to In The Time Trap
The Doctor travels with Daring Do to Saddle Arabia in the year 913 C.E. There they find that Ahuizotl, Daring Do's greatest enemy, is the king, and is oppressing the city with his Royal Guard, that have been given terrifying powers. They decide to help, but that's going to be difficult, especially with the secret of Ahuizotl's power being used against them.