As the royalist forces retreat following a defeat, a group of them remain behind to convalesce at the estate of a local widow. The soldiers have become aflicted with yellow fever I believe, and as the rain continues to pour outside, day after day, they are slowly dying. The widow whose estate they stay at knows enough about medicine to take charge of the care of the soldiers, although there is not much she can do to halt the effect of the fever.
It turns out that her loyalties lie elsewhere, and though she doesn't neglect the soldiers, she lets her views be known. The sergeant, in charge of the remaining eleven soldiers, sees no recourse but to punish her. This sets up the miracle from which the story gets its name.
Sort of with the last one, the strongest element struck me as the grim atmosphere. The conflict would be more dramatic if Lugones had better skill at characterization.