Euripides' Electra (Greek: Ἠλέκτρα) is sometimes thought to have been produced in around 413 BC, at the time of the Sicilian Expedition, but may be somewhat earlier. The play gives Euripides' version of the story of the Argive princess Electra and her long-lost brother Orestes, and their murder of their mother Clytemnestra in revenge for the death of their father, Agamemnon.
This episode was also told in Sophocles' play of the same name and Aeschylus' The Libation Bearers. Euripides seems to satirize some elements of Aeschylus' version, notably with a recognition scene in which Electra rejects tokens of Orestes' identity that had been accepted in the older play.
Free online texts
Gutenberg: Electra, translated by Gilbert Murray (1905). Multiple formats.
Gutenberg: Ηλέκτρα, Greek text. Multiple formats.
Internet Archive: Alcestis and Electra, translated by T.A. Buckley (1900). Multiple formats.
Internet Classics Archive: Electra, translated by E.P. Coleridge. HTML and TXT formats.
Perseus: Electra, translated by E. P. Coleridge. HTML and XML formats.
Poetry in Translation: Elektra, translated by George Theodoridis. Multiple formats.
University of Adelaide (Internet Archive): Electra, translated by E.P. Coleridge.
Gutenberg: Euripides and His Age, by Gilbert Murray.
History of Ancient Greece: Euripides at War, podcast by Ryan Stitt.
Librivox: Electra, translated by Gilbert Murray. Public domain audiobook.
Wikipedia: Electra (Euripides play).
The Great Conversation: Further reading at Tom's Learning Notes
A.E. Haigh: The Tragic Drama of the Greeks (1896).
Ancient Greek resources: Learn to read Greek classics in the original.