Wordsworth and his sister, Dorothy, wake up early to catch a coach to the port of Dover, where they will cross over to Calais, France. As Dorothy wrote in her journal, the time is around 5 or 6am. We can guess that the only others awake were the working-class laborers, beginning their long, arduous day. Everyone else is safe in his or her bed. The streets are mostly empty, and there's no traffic to hold them up. But when Wordsworth and his sister cross the famous Westminster Bridge over the Thames River, they can't resist getting out of their coach to marvel at the scene. Though these details aren't captured in the poem itself, this is how we imagine the scene.
Unlike many a damp London morning, there is no fog, and the sky seems airy and spacious. The sun has begun to rise, casting a bright yellow light over those famous London landmarks. As the sun moves from the horizon, the buildings begin to glitter, as do the innumerable ships docked along the crowded river. The light makes London appear to be a completely different city.
In the second half of the poem, the speaker reflects on other times when he has felt a similar sense of peace and wellbeing. He thinks of his explorations around the English countryside, with its many green hills and valleys, but he decides that even these cannot compare with the vision before him.
Suddenly the city turns into a big sleeping body. The speaker can almost see the expansion and contraction of the houses, as if they were taking deep breaths. This usually vibrant city is calm, for once. The impression is made even more touching by speaker's knowledge that, in a few hours, all will be bustle and hubbub once again.