The play En la ardiente oscuridad centers around a young man, Ignacio, who is admitted to an institute for the blind, managed by Don Pablo. Everything there is so perfectly arranged that the students do not mind the fact that they cannot see. Ignacio, however, always refuses to accept his blindness and struggles to find his way around.
He meets a group of blind residents who appear to be happy, but his sense of dissatisfaction at losing the most marvelous of the senses is contagious and spreads through the group. Carlos, one of the students at the institute, attempts to ease his depression but does not succeed. Carlos is rightfully suspicious of the friendship between his girlfriend Juana and Ignacio, who ends up seducing her. (Juana feels more compassion for Ignacio than anyone else.)
One fateful night, Ignacio and Carlos are arguing. Ignacio tries to convey to Carlos what a blind person who longs to see feels. The idea disturbs Carlos profoundly. Don Pablo and Doña Pepita walk in. Ignacio goes outside to the playground. The remaining three talk for a while and reaffirm the sentiment that Ignacio must leave the institute. Carlos then also goes out to the playground.
Pepita idly stands by the window when she sees something that horrifies her. Suddenly, Ignacio is brought in by other boys, who lay him on the sofa. He is dead. The immediate assumption is that Ignacio committed suicide or had an accident. In reality, he was murdered by Carlos because Carlos could not stand Ignacio’s forcing him to face reality. Doña Pepita admits to Carlos that she knows it was he who murdered Ignacio, but she can’t prove it. She says, “It occurred to me to get up and go to the windowsill. I didn’t do it. If I had, I would have seen someone climb the stairs of the slide with the body of Ignacio, unconscious or dead. Then, without stopping to think about anyone possibly seeing the act, Carlos would let the body fall from atop the slide. But I didn’t see anything, because I didn’t get up to look…”
The uncertainty of what happened is never resolved because the students at the school seem content with the ruling that it was an accident. It is only then that Carlos realizes he is more affected by being blind than he was willing to admit, and the play ends with this realization.
The play may be seen as an allegory about the state of Spanish society after the civil war, unable to see clearly and participate in what is going on around them.