In this short story, two astronauts are exploring an art museum on an abandoned planet. The only living things present are the flies/gnats which swarm outside the museum. They are confused by the art filling the museum, all except one painting at the bottom of the stairway, near the entrance. They puzzle and think about what turn out to be profound truths they find in the work.
As they revel in their discovery, they notice the next painting up the stairs. It suddenly appears more meaningful, in the light of having figured the first painting out. They begin to pore over the next work. Soon we see that they have grown and become more sophisticated by learning from the progression of museum pieces. They have even miniaturized their space packs and can fly about as they continue to study, learn and grow.
The story ends after the astronauts turn into flies or gnats, having progressed through all the museum and evolved completely. They no longer resemble humans, but are flies who join the swarms flying about the planet.