The first stanza of the poem describes the ongoing misery, deprivation, and physical pain of the soldiers fighting in the trenches during World War I. The second stanza, beginning with the line: "Gas! GAS! Quick, boys!" involves a shift of pace and focus from ongoing misery to the sudden panic ensuing from a mustard gas attack. From when the shells are spotted, the soldiers have very little time to equip their gas masks and avoid inhaling the mustard gas. In the poem, most of the soldiers manage to put on their masks in time, but one is too slow and ends up inhaling the gas. 

The third and fourth stanzas describe in excruciating detail the effect of the gas on that soldier. Although his comrades manage to get him in a wagon which will convey him to where he can be treated (although full recovery in cases of severe exposure is not likely), he meanwhile is suffering from agonizing burns of his face, eyes, and lungs. Owen describes him:

white eyes writhing in his face,

His hanging face, like a devil’s sick of sin; ...

the blood ... gargling from the froth-corrupted lungs, 

This vision prompts Own to say that anyone who had the experience of watching this happen would not describe such injuries or deaths as "sweet and proper".