The speaker of the poem has barely gotten to his tomb ("was scarce adjusted') when another person arrives and is buried "in an adjoining room." (Dickinson often uses room as a euphemism for grave.) The newcomer asks the speaker how he got there--what caused his death. The answer is "for beauty"; the asker says he died for truth. He then makes this commentary:
"...the two are one;
We brethren are," he said."
The two of them are next described as "kinsmen," and they talk through the eternal night of death,
"Until the moss had reached our lips,
And covered up our names."
The plot is fairly uncomplicated; it's the concept of truth and beauty being one and the same which generally opens this work to some speculation. What is clear is that each of them died for a worthy cause--truth and beauty. What's also true is that, in the end, the ravages of time (represented figuratively by the moss) affect them both. Whatever the cause, then, noble or not, death is the great equalizer.