3 ideas
A middle-aged brother and sister live idle, unmarried lives in their large ancestral Buenos Aires house, which they keep clean each morning. One night the brother hears muffled sounds from the back rooms and bolts the connecting door. The siblings adapt without investigating, and when the noises reach their side too, they walk out with only the clothes they wear, lock the front door, and throw the key into a sewer drain.
Because the intruder is never named, described, or confronted, the reader's attention moves from what the threat is to how people respond to it. The siblings' choice to accommodate each loss and never ask is what the story actually shows. Leaving the source blank turns the fantastic into a measure of how much people will give up to keep their routine intact.
Cortázar describes impossible events in the same calm, detailed tone as knitting and housekeeping, and he never marks a boundary between the realistic and the fantastic. The absence of alarm in the prose makes the strange feel like part of ordinary life, not a rupture of it. This lets the reader absorb the event before noticing that it is impossible.