It's the middle ages and Earth has a visitor from a Star Warrior named Linx. Linx is actually a Sontaran warrior with a crippled ship. He requires immediate aide from the locals, but finds they are too primitive and their supplies are limited. Irongron is one such local who trades Linx housing and help in exchange for weaponry. However, their trust in each other is leery at best.
Since Earth does not have the proper technology to help Linx repair his ship, he is forced to look to the 20th century for advanced help. He is able to steal equipment and scientists and bring them back in time to help him. When the Doctor is brought in to investigate the missing scientists he discovers what Linx is up to and uses the Tardis to travel back in time to investigate.
The Doctor steps out of the Tardis and walks away but soon Sarah Jane Smith also walks out. She is a journalist who walked into the Tardis looking for one of the missing scientists. As she steps out, she is surprised the Tardis has moved but is unaware she has also travelled back in time. She immediately runs into Irongron and mistakes him and his castle for a film set or medieval themed park.
Irongron admires the 'fire' she has but soon tires of her incessant prattle. He finds it difficult to get information from her when he tries to discover who she is, so Linx uses a device to 'open her mind' and learns she is from the future and has travelled back with aid of the Doctor. Linx discovers the Doctor is a Time Lord but refuses the Doctors help to repair his ship as long as Linx leaves and destroys the weapons he has supplied to Irongron.
The Doctor is able to overpower Linx and free the mesmerized scientists and send them home but soon Irongron comes and sets Linx free. But Linx and Irongron have a disagreement and Linx kills Irongron. His ship repaired, Linx prepares to take off but is killed by Hal a friendly local archer who shoots him in the 'probic vent' - a Sontarans only weakness. As the Doctor and Sarah escape, his ship explodes destroying the castle and all the advanced weapons.
WHO: Sarah Jane Smith
PLANET OF ORIGIN: Earth, circa 1974
REASON FOR JOINING: She's a journalist looking for a story. The Doctor has many to provide her with.
Next: There's Something On Your Back