What is a good summary of the poem "The Solitary Reaper."

In "The Solitary Reaper," the poet (or speaker) describes his experience seeing a Highland (Scottish) girl reaping grain as he listens to her beautiful song. He notes, in the first stanza that the Vale (valley) overflows with the sound. In the second stanza, the poet notes how he appreciates her song more than weary travellers welcome the notes of a nightingale. In the third stanza, he says: 

Will no one tell me what she sings? -

Perhaps the plaintive numbers flow

For old, unhappy, far-off things,

And battles long ago:

She sings in Erse, the Gaelic language of Scotland. He doesn't understand the language. It is exotic and this adds to the mystery and wonder of the sound, almost as if her singing is part of nature. In the last stanza, the poet is so affected by this experience that the song stays with him "Long after it was heard no more." Therefore, the reaper's song transcends time and space; it stays with the poet, causing him to think of how this maiden's melancholy song is part of the ongoing story of nature and humanity. This is a common technique in Romantic poetry. The poet experiences a common, everyday experience but imaginatively integrates the experience with a more universal meaning. 

The poet uses a technique called apostrophe. He addresses an abstract quality, idea or a person who is not present. This speaker might be addressing the reader, himself or nature itself as he begins the poem with "Behold her, single in the field." Also, it is implied that the girl has no idea he is there, so she does what she would do naturally as if no one were watching. This underscores the idea that her song and nature are one.