Everyone congratulates Socrates at the conclusion of his speech, when they are suddenly interrupted by loud knocking at the front door. Soon thereafter, Alcibiades bursts in, drunk, supported by a flute-girl, surrounded by a few other revelers, and wearing a thick garland of ivy, violets, and ribbons. Alcibiades asks if he might join the symposium even though he is very drunk, or if he should just do what he has come to do--to wreath the victorious Agathon with his garlands--and go away again. The company urges Alcibiades to stay, and so he stumbles on to the couch next to Agathon, displacing Socrates.
When Alcibiades notices Socrates, he cries out, protesting that Socrates has been sharing a couch with Agathon, the most attractive man in the room. Socrates asks for Agathon's protection, pointing out that ever since he began loving Alcibiades, Alcibiades has been so jealous of him that he cannot stand it when Socrates comes near another attractive man.
Alcibiades then unties some of Agathon's garlands, placing them on Socrates, since Socrates always beats everyone in verbal contest, while Agathon has only been victorious once, with his recent tragedy. Next, he takes charge of the revels, electing himself master of ceremonies, and insisting that everyone get drunk. He quickly downs four pints of wine, and insists that Socrates do the same, though he observes that alcohol never has any effect on Socrates.
Eryximachus demands that Alcibiades take part in the symposium and give his own speech in praise of Love. Alcibiades complains that this would be unfair, as he has already drunk far too much. Besides, Socrates would get jealous if he were to praise anyone other than Socrates in Socrates' presence. Thus, with Eryximachus' permission, he undertakes to deliver a eulogy to Socrates.
Alcibiades claims that Socrates is like a satyr, both in appearance and in other ways as well. Marsyas, the flute-playing satyr, could produce bewitching flute-music with the power of his mouth. Socrates, Alcibiades suggests, can also bewitch people with his mouth, though he needs no flute, using words alone. Unlike any other orator, Socrates has made Alcibiades dissatisfied with his way of life, convincing him that in following politics he neglects himself. Further, Socrates is the only person who has ever made Alcibiades experience shame. Whenever Socrates speaks to him, he cannot argue with anything Socrates says. But when Socrates leaves he gets caught up in the admiration of others and ignores Socrates' wisdom so that when next he sees Socrates he is once more ashamed.
Alcibiades, in his sudden and unexpected appearance, stands as a thinly veiled symbol for the god Dionysus himself. Alcibiades appears garlanded, young, beautiful, and drunk, much the way Dionysus is often depicted. He is coming in from a street-party, and is accompanied by many revelers, just as Dionysus is frequently accompanied by his Bacchae. Further, Alcibiades urges the guests to drink, and Dionysus is the god of wine.
The symbolic importance of Alcibiades as Dionysus works on a number of levels. Most importantly, he appears at the end of the drama to celebrate Socrates. The dramatic festivals were given in honor of Dionysus, and Agathon's victory would thus have been an homage to him. It is significant, then, that Alcibiades enters in order to wreath Agathon with garlands as the victor at the festival. But when he sees Socrates, he insists that he take some of Agathon's ribbons and place them on Socrates, since Socrates has a way with words that outmatches anyone's. In this sense, we could see Socrates as stealing Agathon's victory from him by besting him in the series of speeches. At 175e, Agathon suggests that Dionysus may judge who is the wiser between them, and, in the form of Alcibiades, Dionysus does just that at the end of the dialogue.
The dialogue itself, of course, is also a drama, and Alcibiades' appearance at the end could be read as the appearance of the god to celebrate the drama that has just taken place. By having Socrates best Agathon, Plato suggests that philosophy is greater than tragedy, and Socrates' celebration by this symbolic Dionysus suggests that the philosophical drama we have just read is greater than any tragic drama. Also significant is the fact that Alcibiades/Dionysus enters after the speech of Socrates/Diotima, missing the final mysteries which Diotima reveals. Perhaps these gifts are reserved only for philosophy, and tragic drama, no matter how good, is bound to miss them.
Finally, we might also note that in the speech of Diotima, Socrates suggests that the gods most love one who knows true Beauty. Here, the god Dionysus seems to be valuing Socrates' wisdom above that of all the others.
There is also a great deal of importance in Plato's choice of Alcibiades the man, outside his symbolic role as Dionysus. Alcibiades was a great politician who was involved in a number of scandals that led to Athens' loss to Sparta in the Peloponnesian War. Socrates was a known associate and friend of Alcibiades and a few other Athenians who had fallen out of favor, and it is suspected that these associations were the real cause of his trial and execution. In order to redeem Socrates' name, Plato carefully points out that Socrates was not responsible for Alcibiades' political misdeeds. Alcibiades notes here that Socrates was always persuading him to abandon politics and to concentrate on his ethical development, but that Alcibiades refused to listen. Thus, Plato renders Socrates blameless of anything Alcibiades may have done in the political arena.