To a Skylark
"To a Skylark" doesn't exactly have a plot. You might want to think of it more like a bunch of observations about a single idea—a stretched-out description of the song of a bird. The poem opens up with the speaker calling out to a bird (which he calls a "Spirit"). He tells the bird how much he loves its singing. Then he describes how it shoots up into the sky at dusk, into the purple evening.
After that, he compares the bird's song to a bunch of different things, including a star, the planet Venus, a poet, a maiden, a worm, a rose, and so forth (yeah, seriously, a lot of things). Then he starts to talk about how all of the beautiful things that human beings make can't compare to the song of this bird. All human songs are sad, but this bird's song is just pure joy. Finally the speaker dreams of being able to sing with as much joy and freedom as this happy bird.