Synopsis: The Doctor and Jamie manage to outdistance the Quarks and hide in the castle's torture chamber. There, Jamie has time to explain how he came to be in the castle.
Jamie had been visiting a friend at a tracking station a half mile from the castle. They picked up the approaching Quark vessel, then the station was destroyed as the ship landed.
Believing himself the only survivor of the Quark attack on the station, he stumbled towards the castle. He was hoping to gather intelligence on the number of Quarks that had landed, so he could alert authorities to the scope of the invasion force.
As he gets to that point in his story, Jamie is interrupted by a Quark smashing through the door to the Torture Chamber . . .
Notes: This "explanation" is really quite more confusing than leaving things mysterious. It raises a number of different questions that are never answered. Why would the Quarks base an invasion of Earth in remote Scotland? What year is all this set? Clearly it must be at least after radar was invented, but since this 2-panel flashback is the only thing in this story set outside an ancient castle, it reveals very little about the year. How does Jamie have a friend who's ostensibly the lead scientist at a (probably) top secret tracking station? How did Jamie become separated from the Doctor prior to this encounter? How did he end up obviously displaced from his own time?
Roger Noel Cook has a lot to answer for on this one . . .