The speaker of "A Dream" recalls an experience that he suspects was only a dream.
He and someone named Martin were sailing down a stream, passing through a forest glade as pine branches and the sun hung overhead. Trees rose from beautiful nearby mountains, and notes of "wild pastoral music" filled the air.
Perched upon mossy rocks at the edge of the stream was a lovely cottage. The speaker watched two beautiful women, wearing straw hats with blue ribbons, walk out onto the cottage balcony. The women, who were Olivia and Marguerite, waved at the men. The poem is addressed directly to the latter woman.
The couples gazed at each other for a moment before the "River of Life" pulled the vessel away and the cottage faded from sight. The men then passed "burning plains bristled with cities," and finally ended up in the ocean.
This poem can be interpreted in a number of ways. Because it is so explicitly described as a "dream" and has so much evocative imagery, one could find several ways to parse its meaning. However, it is overall best understood in terms of Arnold's common themes of anti-modernism, nature, and love.
The pastoral landscape is unmistakable. The speaker and Martin begin their journey in a beautiful forest glade — as usual, beauty is represented through nature, which Arnold presents as pure and steadfast. Equally clear is the disappointment they later find, when that landscape turns into "burning plains bristled with cities," or in other words, a modern world tainted and changed by society's overbearing enhancement. Leaving little room for the metaphor to be misunderstood, Arnold titles the stream "The River of Life." It reflects that way that life has transformed even within his lifetime from a natural, beautiful place to one overrun.
The most central episode of the poem is the brief encounter with the two women, which can be interpreted along these lines as well. The women beckon to the men from a gorgeous little cottage, offering them a reprieve from the swiftly-moving river of life through romance. In Arnold's work, he usually suggests the possibility of transcendence that is nevertheless impossible to fully realize. In this case, life (the river) simply moves too quickly, prohibiting them from appreciating the world's full potential. They have glimpsed the possibility of escape, but are not able to take advantage of it. The importance of the moment is reflected in the fact that the speaker addresses the poem to Marguerite, one of the women. Clearly, that moment helps him contextualize his life better than any others, even if he was hardly able to take advantage of it. This fact suggests the speaker's poetic, romantic instinct.
The juxtaposition of water and fire is also a key element of this poem. For Arnold, water represents serenity. He describes how peacefully and beautifully the stream flows through the forest glade in the beginning of the poem, and later reaches tranquility only when at the mercy of the sea. It is the "burning plain," or fire, that symbolizes the bad, the negative, the things that are wrong with the world. Since, as a poet, Arnold pays such close attention to nature, it is fitting that he incorporates much of the elements — earth, air, water, and fire — into his verse.
Finally, this poem reflects Arnold's usual spiritual, almost religious, sensibility. If the river represents life, then the ocean - overwhelming and nothing but water - can easily be read as the heavens. As usual, Arnold does not make an explicitly religious point, but does dream of another realm where he might be at peace. Having gone through a river of life that briefly promised transcendence but mostly offered fire, he only comes to peace when the river ends and he is at the mercy of a serenity greater than he has yet known.