It's chore time, everyone. Now pick up your rakes and your shovels and head on out to the field. At least, that's what our speaker is doing at the start of the poem. He's off to turn over a field of mown grass. Someone—we're not told who—has used a scythe to cut down a whole field of grass in the early morning. (Remember that this poem was written in the days before riding lawnmowers. Check out "In a Nutshell" for more.)
When the speaker gets to the field, he finds that the mower has finished his (or her) work and left. There's no trace of anyone, which prompts the speaker to reflect how each of us is ultimately all alone in the world. Cheery thought, right?
But wait—suddenly a butterfly catches the speaker's attention. It's flitting around in search of a flower to land on, the one it enjoyed yesterday. Sadly for the butterfly, though, that flower's been cut down. Things seem twice as bad now: the speaker is all alone and the butterfly has nowhere to hang out and do its butterfly thing.
The speaker thinks of "questions that have no reply" (19), probably along the lines of "Why is life so hard for me and this butterfly, out here on our own?" Then he decides to get back to work.
But wait again—the butterfly catches the speaker's eye, and this time the speaker notices some flowers near a brook. He walks over to them and determines that they're a kind of flower called "butterfly weed." He decides that the mower didn't leave them there for him to discover or to attract attention to himself (or herself—it's not clear which). Nope, the mower left them there in an act of "sheer morning gladness" (30).
The discovery has a deep effect on the speaker. He can hear the birds now, and he imagines the sound of the mower's scythe moving through the grass. He feels a connection to this other person, so much so that, when he takes his lunch break at noon, he imagines having a conversation with the mower. In it, the speaker totally changes his mind about his isolation. He tells the imaginary mower that "men work together" (41).