The name of the story comes from the serenade that a payador/montonero sings for a landowner in order to convince him (the landowner) to give him and his buddies some aid in the war. The plot is that the payador shows up with two fellow montoneros at the hacienda of this patriotic landowner. The song gets performed, and the landholder gives the men fresh horses, provisions, and a firearm. This one appears to involve mostly subtle touches.
There's the usual atmospheric motif, which struck me as rather enchanting in this story. It didn't really add to the plot but Lugones' use of imagery and metaphor was striking.
There's also some interesting character sketching, both with the payador and the landholder. The landowner is fixated on the book Historia de Carlo Magno y los Doce Pares de Francia, which I believe translates into The History of Charlemagne and the Twelve Pairs of France. From the little we're told about the novel, it appears to be a rather sensational telling of the life of Charlemagne and includes feats of derring-do against creatures like giants. When someone tries to tell him the book is largely fiction, he gets angry and refuses to believe it.
When the payador shows up and asks him for aid, the landholder gets excited and starts talking about the characters in the book. The payador only understands that the landowner is talking about great men who are long dead and infers that the old man is talking about people he knew in his lifetime. So, there's some shades of Don Quixote there, though with Lugones' own variation on it.
(Due to sleep deprivation, I had to read this story twice and ran out of time to write it up. I'll be doing two today.)