The story begins with Roger Mycroft, a veteran of the Spanish-American War, visiting Monsieur Toussaint, a voodoo spiritualist who purports to speak with the dead. Initially skeptical, Mycroft is convinced by Toussaint's serious demeanor and agrees to return the next evening.
Back at home, Mycroft embarks on a long flashback to the his time during the war when he and the soldiers in his squad were guests of a rich landowner, Don Jose Rosales y Montalvo and his beautiful daughter, Juanita. Over the next few days, Juanita is courted by every one of the soldiers and refuses them all in turn. On the soldiers' last night in Cuba, Juanita promises to marry the last surviving man, as so to spare the feelings of his fellows . The soldiers form a tontine and Last Man's Club* and meet once a year to reminisce (and, presumably, to see who's died in the interim.) After many years, Mycroft is the last man alive and can marry Juanita.
Mycroft returns to Toussaint to have him raise the spirits of his fellow soldiers. He discovers that she too has died in the intervening decades and appears to him. She lures Mycroft out of the protective hexagram that Toussaint has drawn on the floor. Mycroft falls dead and joins his demon bride.
Weird Tales, May 1950, v. 42, n. 4
*While the tontine and the Last Man's Club are often conflated ideas, the military veteran Last Man's Club is a pact made to meet once a year often with the agreement that the last surviving member drink a toast at the last meeting to his deceased comrades, and thus differs from a tontine in that there is no prize or payout.