Here is a stanza-by-stanza summary of Wordsworth's early nineteenth-century poem.
1. The speaker invites passers-by to turn their attention to a young Scottish woman working alone in a field. She is harvesting grain and tying it into bundles, and as she does so, she is singing a sad song that fills the valley. The speaker advises that people should either stop and listen or pass quietly.
2. The speaker observes that her voice and song were beautiful and more welcoming than a nightingale's song would have been to nomads in the deserts of Arabia or a cuckoo's song in the spring that breaks the quiet isolation of Scotland's westernmost islands.
3. The speaker speculates on why her song is so sad. He wonders if it is inspired by ancient battles or tragedies, or something more everyday—perhaps a loss or grief that is an inevitable and repeating pattern in life.
4. The speaker says that he stood and listened to her song as she continued to work in the valley. As he moved on and began climbing the hills surrounding the valley, he carried the memory of her song with him, long after he could actually hear it.
In "The Solitary Reaper," a young Highland girl is reaping alone and singing to herself while she works. The poet asks us to listen carefully, because the whole valley is filled to overflowing with the sound of her singing. We are invited to either stop and listen to her sad song or to gently pass by, so as not to disturb her.
He goes on to say that no nightingale ever sang a more welcoming song to bands of tired wanderers in an Arabian desert. The poet asserts that the beauty of the girl's singing exceeds that of the cuckoo bird's in the spring-time. Her singing is also the only thing which breaks the silence of the seas among the Hebride islands. The Hebrides are a group of islands (consisting of the Inner and Outer Hebrides) off the north-west coast of Scotland.
The poet begs someone to tell him what the girl is singing about. Receiving no answer, he muses that, perhaps, she is singing about old sorrows, or battles of long ago, or more mundane concerns, or even some suffering which she has endured and may endure again.
He eventually resigns himself to the fact that he may never find out the theme of her never-ending song. Nevertheless, he asserts that her singing has so captured his imagination that he will bear it in his heart long after it is heard no more.