We begin with a speaker who is weeping. For whom does he weep? Well, it's for Adonais, the Greek god of beauty and desire, who has just died as the result of another god's jealousy. The speaker calls on Urania, Adonais' mother, to arrive at his deathbed. After she arrives, the speaker spends some time admonishing her for not intervening and saving his life. After he's done, he summons nature, some more Greek gods, and some of the Romantic poets to join them. He wants everyone there to mourn the loss of the beautiful youth.
After a long period of mourning, the speaker says that the time for weeping is over. The youth has gone to a better place, one where he is part of nature and eternity forever. His concept of the afterlife, which includes joining with a spirit or force that is contained in every living thing on earth, means that the youth isn't really dead. Therefore, he reasons, there's no reason to cry.
What's more, he says, the youth's work lives on. Still, other mourners show up, including famous poets of years past who have been dead a long time. The speaker reasons that Adonais will join them as their King in the afterlife, since those who think lofty thoughts deserve the highest honors.
Finally, the speaker instructs the mourners to visit Rome, where Adonais is buried. There they see the ruins of the fallen empire and the beauty of the city, and finally come upon the field where the body rests. It is beautiful there, and he thinks it will comfort them. The speaker, however, hasn't been fully consoled. Now he thinks that he should join the youth in the afterlife, because this world just isn't good enough anymore. The poem ends with the final image of the speaker's soul seeing the young man's spirit in the heavens.