The play opens with Heracles' wife and children at the altar of Zeus in Thebes, where they are threatened with death by the tyrant Lycus. Heracles returns from the underworld, where he had been engaged on the last of his twelve labours, and rescues them. However, the goddess Hera drives him mad and he kills his own family. When he recovers his senses, he is filled with despair. Consoled by Theseus, who had returned from Hades with him, he is taken to Athens to be purified.
Free online texts
Internet Archive: Euripides with an English translation by Arthur Sanders Way (1930). Vol. III. Bacchanals, The Madness of Hercules, The Children of Hercules, The Phoenician Maidens, Suppliants. Multiple formats
Internet Classics Archive: Heracles, translated by E.P Coleridge. HTML and TXT formats.
Loebulus: L011N - Euripides -- Euripides III: Bacchanals. Madness of Hercules. Children of Hercules. Phoenician Maidens. Suppliants. Greek and English parallel text. Loeb edition, PDF format.
Perseus: Heracles, translated by E. P. Coleridge. HTML and XML formats.
Poetry in Translation: Herakles, translated by George Theodoridis. Multiple formats.
University of Adelaide (Internet Archive): Heracles, translated by E.P. Coleridge. Multiple formats.
History of Ancient Greece: Euripides at War, podcast by Ryan Stitt.
Wikipedia: Herakles (Euripides)
The Great Conversation: Further reading at Tom's Learning Notes
Seneca: Hercules Furens - A Latin play telling the same story with some differences of plot.
A.E. Haigh: The Tragic Drama of the Greeks (1896).
Ancient Greek resources: Learn to read Greek classics in the original.
Bloom's Western Canon: Heracles is listed.