The poem is split into two parts.
The first part begins by describing a calm, tranquil evening out amongst some fields. All is quiet beneath the stars until several drunken youths (the Bacchanalia) suddenly rush out to the fields, ruining the tranquility with their cries and hollers, trampling the grass in their excitement and amusement. The speaker confronts a hypothetical shepherd, wondering why he is not excited by their presence.
The final two lines of part 1 offer that shepherd's reply: "Ah, so the quiet was! / So was the hush!"
The second part of the poem is organized in a similar way. It describes the world as quiet, having just passed through an era. All its poets, artists, actors, and critics have created their work and gone, leaving the world changed behind them. That old age has passed on, leaving only a few immortal traces behind, shining above the sky. Suddenly, the new age rushes in much like the youths did in the first part. This new age brings new rules, new systems, new schools, and new poems, making the once-still world over-excitable. The speaker confronts a hypothetical poet (as he did to the shepherd), asking why he is not excited about this new age.
The poet gives a similar reply as the shepherd did, one that prizes "silence" and "hush." In the poem's final lines, the speaker notes that the world is usually only concerned with its present, while the poet always feels the past alive as well.
This poem yet again offers a criticism of modernism. Both parts of the poem propose a dichotomy of mutually exclusive experiences. The first section of each part describes stillness, the second frenetic, orgiastic excess. Though the second replaces the first, there is an implication that the two phases are cyclical. Sooner or later, the young people will leave the field, and the "new age" will end as the previous one did. As is the case in "Consolation," Time is a tragic force here, one which moves on no matter man's desire.
The only true escape comes from a romantic sensibility, which allows the poet to feel the past while understanding the present. What Arnold means by suggesting that the poet understands both the present and the past is that such insight allows the poet to contemplate eternity, those things that are unchanging, and hopefully to then articulate or discover what in man aspires to such immortality.
This means the poet must recognize both what is good and what is bad. It would be folly to ignore the new age, which in the context of Arnold's work clearly refers to his world's obsession with science at the expense of faith. And yet he feels compelled to remember what was, not only the previous age, but also the calm between them, the time when all was possible. It is unfortunate that experience does not come in moderation - his descriptions veer between tumult and stillness. However, most unfortunate is that the poet is ironically troubled by his special insight. This is a common theme in Arnold's work, the idea that poets (or the romantic types) are troubled by understanding how the world and life might be better.
Arnold develops his theme largely through mythological allusions. In Greek history and mythology, the "bacchanalia" were festivals dedicated to the god of wine, Bacchus. The followers of that god's mystery cult were marked by their excessive and raucous hedonism. The young people who pollute the night's stillness with their party are clearly meant to be this type of group. And yet while many scholars understand the followers of Bacchus as defined by their community, Arnold subtly suggests that they are marked by self-indulgence, blindness to the beauty they disrupt.
This understanding is important, since Arnold then uses the bacchanalia as a metaphor for those who engender the modern world. Ignoring what is beautiful about the world they are changing, these people of the new age destroy without any sense of what they are doing. They are blinded by their modern ideas, to the detriment of what existed before.
Arnold uses a number of other effective devices to underline his theme. The parallel structuring successfully conveys his feelings about the modern world in a much subtler way than he does in other poems. It is clear that he sees the new age as unorthodox and harmful. Further, the repetition of "the famous" in the beginning of part II emphasizes that these authors, poets, critics, and players were characteristic of old society, but now have all run their course. Their fame may have seemed important, but was fleeting. Time has little use for the laments of mankind, leaving poets to accept and then make sense of its passage. Finally, the lines are purposefully shorter in the second section of each part (when the revelers and new age come in, respectively). This shift emphasizes the abruptness of the arrival, as well as the fact that the new entry brings a total change.
As usual, Arnold makes successful use of nature metaphors and imagery. As in many of his other poems, he presents a pastoral scene as transcendent. The pastures and fields of untainted farmland offer prime example of both how the world used to be and how it should be. He describes the night sky as the spirits of the few immortal pieces of old society rise up to look down on the New Age, "like stars over the bounding hill." Time marches on for all men, but nature is ideal and unchanging. Thus, it is only by focusing on nature that we can come closer to what which is transcendent within us. Everything else is temporary and superficial.