The poem begins with a speaker addressing a star. This tells us that the setting is at night. Well, at least the "imagined" setting is – this is a "lyric" poem, so it's more about recounting the speaker's imagination and feelings instead of telling a story. Maybe the best way to think about a lyric poem is as a dream – we all know that there are places or "settings" for the action in a dream, but that these can also change at any moment, and are sometimes unclear. So, yes, the speaker is addressing a star. As we talk about in our section on the "Speaker" himself, the fact that this is the North Star might also be significant for the setting. Because the North Star is typically used for navigation by travelers – especially travelers at sea – this might mean that the speaker is somewhere far from home, maybe at sea. (And if the speaker were at sea, the rocking motion of the water might be echoed in the gentle rising and falling motion of the woman's chest at the end of the poem.)
But this is all just speculation – remember, the speaker doesn't say anything explicitly about where he is. It's probably best to think about the traveler idea (which we get from the North Star image) as a metaphor for how the speaker is feeling. He might feel like he's adrift, but we don't really know where he is. In any case, the setting doesn't stay with the speaker for long. Instead, our field of vision suddenly rises, like the world's biggest crane shot (or maybe zooming out on Google Earth) from looking at the star to looking down on earth from the perspective of the star. And what does the star see? From that high up, the world is just a collection of barren, desolate scenes: endlessly moving waters wiping away the last speck of humanity, and cold blank snow landing on cold blank mountains and moors.
But then Keats's speaker decides that he doesn't like viewing the world from so high up – instead, he suddenly zooms the perspective back down to an incredibly intimate scene: a man and a woman lying together, the man with his head resting on the woman's breast. In fact, the perspective is so intimate that we don't even get the whole scene; the emphasis is completely on the specific point of bodily contact. This setting, Keats's speaker tells us, is where is would like to spend all eternity. And if he can't he would like to "swoon" away into an entirely different setting, the non-setting or non-existence of death.