Lowell begins his poem "For the Union Dead" by speaking about the South Boston Aquarium, which is an unexpected start when considering the title. He immediately places the reader in the present, which is also perhaps an unexpected setting. The aquarium seems to have been closed for a while; the windows are boarded, and its tanks are dry. Dry tanks in an aquarium are rather odd, so early on, it is established that the reader should expect the unexpected in this poem.

Lowell then flashes back to his own personal past, when he visited this now-closed aquarium, letting his "nose [crawl] like a snail on the glass." He reflects that he still often thinks about these memories of childhood trips to the aquarium. By comparison, he reflects about a time "last March" when he watched some construction happening in Boston Common. He personifies the machines working on the construction; they "gouge their underworld garage" into existence. The parking spaces created are implied to be gaudy in contrast to the world around them; they are "Puritan-pumpkin colored girders," which is a form of women's underwear. Lowell says that things which should be discreet are instead rather obnoxious in this environment.

There is then a shift to the subject the reader anticipated upon reading the title. Colonel Shaw led an all-black infantry in the Civil War. As the construction shakes the ground around Shaw's statue like an earthquake, it seems barely separated from the disarray of machines behind it. Two months after marching through Boston, half of Shaw's men were dead. William James, who appears in the poem, actually gave a speech at the dedication, and he "could almost hear the bronze Negroes breathe."

Lowell then personifies the statue:

he seems to wince at pleasure,
and suffocate for privacy.
He is out of bounds now.

The statue of Shaw seems out of place in this modern context. The ideals he fought for are now being lived out all around him. He exists outside this time, yet is strangely trapped in it too, via the statue.

The speaker notes that the significance of what the soldiers accomplished seems to lessen each year. He notes that Shaw's own father didn't even want a monument to him; all he desired was for his son to be buried with his fellow soldiers.

Back in the present, the speaker notes that there are no monuments to mark the heroes of World War II, inferred from the reference to Hiroshima. Instead, a photograph of Hiroshima is used as a "commercial," in an effort to make money instead of honor heroes. On television, he notes the struggles of African American children but also notes that they "rise like balloons" in the face of adversity.

The monument to Shaw still stands watching over the progress of modern day, waiting for the "break" of true equality in the world around him.