The boy stood on the burning deck
Whence all but he had fled;
The flame that lit the battle's wreck
Shone round him o'er the dead

The poem commemorates an actual incident that occurred in 1798 during the Battle of the Nile aboard the French ship Orient. The young son Giocante (his age is variously given as ten, twelve and thirteen) of commander Louis de Casabianca remained at his post and perished when the flames caused the magazine to explode.

[edit] Narrative
In Hemans' and other tellings of the story, young Casabianca refuses to desert his post without orders from his father. (It is sometimes said, rather improbably, that he heroically set fire to the magazine to prevent the ship's capture by the British.) It's said that he was seen by English sailors on ships attacking from both sides but how any other details of the incident are known beyond the bare fact of the boy's death, is not clear. Hemans, not purporting to offer a history, but rather a poem inspired by the bare facts, writes:
Yet beautiful and bright he stood,
As born to rule the storm;
A creature of heroic blood,
A proud, though childlike form.
The flames rolled on—he would not go
Without his Father's word;
That father, faint in death below,
His voice no longer heard.
Hemans has him repeatedly, and heart-rendingly, calling to his father for instructions: "'Say, Father, say/If yet my task is done;'" "'Speak, father!' once again he cried/'If I may yet be gone!;'" and "shouted but once more aloud/ 'My father! must I stay?'" Alas, there is, of course, no response.
She concludes by commending the performances of both ship and boy:
With mast, and helm, and pennon fair,
That well had borne their part—
But the noblest thing which perished there
Was that young faithful heart.