"The Second Coming" is narrated by a speaker who is observing the world around him with horror. The poem begins with the phrase "Turning and turning in the widening gyre," a sentence that evokes an occult symbol that perpetually fascinated William Butler Yeats: interlocked circles. A gyre is a spiral or vortex, and Yeats believed that the universe was comprised of interlocked circles, which together make up up individual lives that coalesce to form the whole of existence.
Essentially, this first line is just a complex way of saying that something is happening in this world. Something is churning and awakening; some new existence is rising out of the current haze of life that we all live in, expanding it and enlarging the scope of what life is and altering how the world works on a fundamental level.
The whole first section finds the speaker observing a world that is losing touch with order and morality. Violence is destroying innocence, people have become detached from their leaders, something fundamental is dissolving, and people who believe in goodness are being silenced, while the loudest speakers are the villains and chaos-bringers.
The second section, beginning with the line "Surely some revelation is at hand," finds the speaker sure that some major shift is happening around him. All this chaos cannot be an accident, certainly. Something vast is coming, some distorted version of the Christian apocalypse is descending upon the land; some ending is approaching.
The third section describes the speaker's vision for what this Second Coming, this new world redefined by all the violence and chaos that occurred in the past, might look like. He thinks about the "Spiritus Mundi," which is a Latin term meaning "World Spirit," and begins to visualize images within this "World Spirit," including desert sphinxes and shadowy birds.
By the end of the poem, the speaker is sure that something even worse is coming. Some nightmare—some "rough beast"—is rising, approaching the earth at a rapid pace. He doesn't know what this creature is, but he can sense its approach—and it is the ominous core of "The Second Coming," that mysterious tide of evil and mystery approaching the world in the form of a modernity full of violence, war, and the loss of traditional meaning and values.