Narrator, along with Jerome Glass & Sammy Cross, has traveled to Pluto on an exploration mission when Pluto was closest to earth. About 18 months one way trip.
Sammy will stay in orbit; narrator with Jerome goes down in a landing craft.
Something goes wrong after landing. I didn't quite catch all the details of accident, but the two are now marooned. No way to go back to orbiting return vehicle.
Sammy quickly commits suicide - by removing his helmet, & quickly freezing to death.
Narrator does something similar but doesn't die. He is now in a semi-stasis. Comes alive with very sluggish metabolism & thoughts during night; is pretty much dead during the day.
Story is told by this frozen narrator for whom the time has all but stopped. And he is awaiting rescue which will surely come some day!