Dionysus is the son of a mortal woman, Semele, and the male god, Zeus, whose jealous wife, Hera, tricked him into killing Semele with a lightning bolt when she was pregnant with Dionysus. After striking her down, Zeus hid Dionysus in his thigh until the god was ready to be born. After becoming popular in Asia, Dionysus returns to Thebes to take revenge upon his mortal relatives, his mother's sisters who defamed her, and his cousin Pentheus, newly appointed king, who has forbidden worship of Dionysus in Thebes. When Dionysus arrives in Thebes with his frenzied followers, the Maenads, he has already possessed his mother's sisters, Agave, Ino, and Autonoe, turning them into frenzied Bacchae and sending them into the forests of Mount Cithaeron to perform mysterious Dionysian rituals. Pentheus returns from a trip and, discovering the women of Thebes have gone mad worshipping the god, determines to capture Dionysus and imprison or kill him, as well as the women. However, Dionysus is a powerful god, and he easily tricks Pentheus and exacts revenge upon the disrespectful and the disbelieving. Dionysus, at once both male and effeminate, is joyful and vicious; Dionysus is not just a merry god of revelry, but savage, excessive, and irrational.
Cadmus has stepped down from the throne to hand the kingdom over to his grandson, Pentheus. Unbeknownst to Cadmus, the stranger who arrives in Thebes is his other grandson, the god Dionysus. Cadmus, at the urging of his friend, Tiresias, embraces the new religion that breaks out when Dionysus arrives, but Cadmus is punished anyway, because Dionysus still blames him for what happened to Semele.
Pentheus, a young king, son of Agave and cousin of Dionysus, is opposed to the new Dionysian religion spreading throughout Thebes, although he finds the new religion intriguing. To Pentheus Dionysus is chaotic and unbound by law. Law, the basis of social order, must be preserved. When Pentheus sees his mother and aunts have been completely overcome by wild ecstasy, he is afraid for what this new religion has done to them, but also what it may do to the people of Thebes—particularly women, over whom he does not want to lose control. He determines to imprison and punish Dionysus and his followers, the Bacchae. Unfortunately for Pentheus, Dionysus lures him into the religion figuratively and literally, leading him to Mount Cithaeron, where the Bacchae, at Dionysus's command, rip Pentheus to shreds, and the king dies, paying the ultimate price for refusing to believe in the power of the gods.
Agave, mother of Pentheus, daughter of Cadmus, and sister of Semele, Autonoe, and Ino, does not speak until the end of the play. Several messengers arrive onstage to speak of how she and her sisters, turned into frenzied Dionysian followers called the Bacchae, have run off to the forests of Mount Cithaeron to engage in orgiastic rites and a variety of unusual behaviors, from hunting with their bare hands to suckling woodland creatures. Agave has been taken over by the god Dionysus, who is angry with her for rejecting his divinity and telling a lie about his mother, Agave's sister, Semele, who died before Dionysus was born. In a demonic Dionysian trance, Agave descends upon her son, Pentheus, and tears him apart, believing she is killing a wild lion. When she comes to her senses and sees she has killed her own son, she is distraught. Dionysus banishes her from Thebes to make her punishment complete. The audience is left to wonder if her crimes against the god and her sister Semele merit the punishment Dionysus metes out.
Tiresias provides several warnings to Pentheus, all of which go unheeded. Although the Dionysian religion is new and from a foreign land, Tiresias and Cadmus agree to make a thyrsus, don the fawn skins and crowns of garland, and set off to Mount Cithaeron to participate in Dionysian rituals. As Tiresias points out in his initial dialogue with Cadmus, no one in Thebes but the two old men (and the women) are embracing the new religion. As Tiresias sees it, only his and Cadmus's thinking is clear. Everyone else is wrong about the cult of Dionysus.