September 1973.
The fledgling UNIT has had a busy summer, tackling Tobias Vaughn and being infiltrated by the alien Jocransten.

Now Brigadier Lethbridge-Stewart finds himself leaned on by the British Government to assist the Spanish army in an investigation, couched as a training exercise in Seville. Major Branwell steps up to the mark to prove his worth, with Corporals Benton and Tracy for support. But it seems that almost everyone has something to hide – even the dead.

What do the lights in the sky above Seville signify? Who can the UNIT team trust? And what is killing people in the darkness?

It quickly becomes apparent that the situation is far more dangerous and involved than the British or Spanish governments let on. UNIT reinforcements are not an option. Besides which, Captain Turner is off leading a mission of his own back in England. Only Benton seems to be enjoying himself, having befriended a local teacher, Pilar Ortega, and her feisty Abuela. But where will that lead when the threat of death is all around?

And why does the local monastery look like a fortress?