Doctor Who: Battlefield (TV)

The TARDIS picks up a distress call from modern-day Earth. The Doctor and Ace discover that a UNIT platoon has come under assault whilst transporting a nuclear warhead. The attackers turn out to be Arthurian knights from another dimension, led by the legendary sorceress Morgaine. The Doctor learns that a future incarnation of himself will become Merlin, and that the location where he buried King Arthur and Excalibur is the key to defeat the demonic entities Morgaine is summoning.