Broadly, the speaker hears the singing of a nightingale. Hearing the bird's song brings both pleasant and unpleasant emotions. The speaker states that he wishes he could drink himself into unconsciousness, in order to, essentially, escape the pain of living. In the absence of wine, the speaker turns to poetry.
He attempts to, in a sense, become the nightingale in his poem. In stanzas five in six, the speaker imagines life as the bird. Returning, then, to an observation of the bird, the speaker mentions the long history of the nightingale and then, as the bird flies away, returns to (harsh) reality.