Electra is a telling of the classical Greek tragedy by the same name. It all starts with background regarding King Agamemnon's murder after his return home from the Trojan War. The murder plot was carried out by no other than his own wife and queen, Clytemnestra. Queen Clytemnestra didn't commit this heinous crime on her own though. She was helped by Aegisthis, both her relation and paramour.
Electra is the daughter of Agamemnon and Clytemnestra. She seeks revenge for her father's murder and enlists the help of her brother Orestes and their cousin Pylades. Orestes lets Pylades into Clytemnestra's House, where he stabs her and Aegisthis to death in their bed.