What is the substance of the poem "Lucy Gray" by William Wordsworth?

“Lucy Gray” is about a little girl who gets lost in a snowstorm; her parents search for her all through the night, but find no sign of their daughter.  Finally, at daybreak, the mothers spies Lucy’s footprints in the snow.  The footprints lead to the middle of a wooden bridge a short distance from their home; there the footprints end.  Lucy is never found,

--Yet some maintain that to this dayShe is a living...

“Lucy Gray” is about a little girl who gets lost in a snowstorm; her parents search for her all through the night, but find no sign of their daughter.  Finally, at daybreak, the mothers spies Lucy’s footprints in the snow.  The footprints lead to the middle of a wooden bridge a short distance from their home; there the footprints end.  Lucy is never found,

--Yet some maintain that to this day
She is a living child;
That you may see sweet Lucy Gray
Upon the lonesome wild.

The speaker begins the poem with his own sighting of Lucy Gray, upon the moor where she lived and played, at dawn.  Thus we have the dissemination of the girl’s spirit into nature; the speaker sights her, but the sighting is spectral and rooted in tales that “some maintain” – local legends.  In this way, Wordsworth is exalting the imagination and the power of nature to keep the spirit alive, even when all trace of one’s existence has been wiped away.

There is also an emphasis on solitude in the poem – Lucy Gray always plays alone, she goes into town alone, and she dies alone.  And yet there is little to indicate any sadness linked to this solitude – Lucy “gladly” goes to town by herself, and is quite content to play on her own on the moor.  She is a happy child, and makes a happy ghost and a happy tale, and even death cannot dampen the sweetness that was in her, and that remains in her memory.