Doctor Who: The Time Warrior.

When scientists start to go missing in the 20th century, the Doctor is called in by the Brigadier to investigate. His investigations lead him to deduce that they are being kidnapped through time and he sets off in pursuit, unknowingly kidnapping journalist Sarah Jane Smith in the process. Arriving in the middle ages, the Doctor and Sarah find themselves caught up in the machinations of the robber baron Irongron and his man from the stars. The alien, a Sontaran named Linx, is arming him with modern weapons in return for helping him repair his damaged ship, and it's up to the Doctor and Sarah to stop him from ruining the Earth's timeline.