The Sarah Jane Adventures: The Thirteenth Stone

Sarah Jane is helping out on a school trip with Luke, Maria, and Clyde's class. On the way home, the group stop off at the Stone Whisperers, a dozen standing stones enclosed by a dome. Set apart from the circle is a thirteenth stone: the King Stone. Legend has it that this is an evil king who was captured in battle by twelve knights, and turned to stone. To keep him imprisoned, the knights too turned to stone, holding him forever in their power. Nice story, but it's just a myth — isn't it?

Luke finds himself strangely drawn to the King Stone. Over two metres high, and covered in moss, it glows with a strange, unearthly light. But when the secret of the stones is finally revealed, the school party's enthusiasm turns to terror.