Synopsis: In a grand castle, Earth's last line of defense attempts to hold back the horde of Catavolcus and his army. As the Doctor arrives--having been positively summoned--the push is on to prevent Catavolcus from getting his hands on the Dragon, a nuclear reactor.
Of greater interest to the Doctor is who summoned him. An elderly wizard pushes forward, claiming both responsibility and a link to Earth's ancient past. He, too, has come to this spot in time to defend the Earth. Seeing the TARDIS as a means of escape, the wizard overloads the reactor and calls for the troops to abandon their defense and enter the TARDIS. As they try to make their escape, though, Catavolcus's troops breach the castle. The leader of the defenders--apparently a king--lays down his life so that his men can make it to the TARDIS. They do so. Catavolcus' army is completely destroyed in the subsequent explosion.
Sometime later, the Doctor wakes up in a forest, unsure how he's gotten to where he is. All he remembers is that the wizard and the king called each other "Merlin" and "Arthur".
The Doctor sees an image of Merlin, who tells him they will meet again . . .