In the prologue, we have Mercury, messenger of the gods and patron of thieves and tricksters, tell us the performance will be a tragedy, but then he quickly walks back that declaration:
What? Frowning because I said it's tragedy!
I'm a god. I'll change it for you:
Transform this selfsame play from tragedy to comedy and never blot a line.
Later, Mercury says that instead the play will be a tragicomedy, since you don't have kings and gods in all-out comedy. Thus, the genre of tragicomedy was born, blending together the conventions of two different genres. Later authors who wanted to mix genres could point back to Amphitryon as a model for their own experiments.
In the first act, the conquering general Amphitryon has returned from a war against the Teleboians on behalf of the king of Thebes, Creon. He's brought back with him his faithful servant Sosia, but little do they know the god Jupiter has just spent the night with Amphitryon's wife Alcmena. Jupiter has disguised himself as Amphitryon, and Alcmena is ignorant of the deception. What is more, Mercury has disguised himself as Sosia, who gets beaten up by his own doppelganger.
When the real Amphitryon shows up, Sosia doesn't know how to explain what has happened. Things get even more confused when the general sees his wife for the first time in months, but she assures him that they just parted. Sosia compares Alcmena to the same type of ecstatic women who appear in Euripides's play The Bacchae:
These raving maenads when they're raving--
You mustn't cross them...
Or you'll make the crazy things still crazier
And get yourself torn in two.
Alcmena isn't about to tear anyone in two, yet she has been possessed by a god... though in a different way from the maenads who worshipped Bacchus. She shuts herself up in her house, and when Amphitryon tries to get inside, Mercury as Sosia mocks him and dumps water on his head. This scene provided the model for the first scene of Act III in Shakespeare's The Comedy of Errors, though most of that play is inspired by another Plautus work, Menaechmi.
Amphitryon gets resolved when Jupiter miraculously reveals to the general and his household what has been going on the whole time. Alcmena gives birth to two sons, one biologically Amphitryon's, while the other is the child of Jupiter and destined to become the great hero Hercules. The general doesn't mind being cuckolded as long as it was by a god, and he accepts his situation, sending away the prophet Tiresias, who had been called in to sort out the matter.
So many of the characters--Creon, Tiresias, Hercules--appear in other ancient tragedies we have, and apparently the tragic dramatist Sophocles even wrote a play about Amphitryon, though it is now lost. Overall, though, Plautus's Amphitryon presents the action in a decidedly comic manner.
