Seneca's Thyestes was written in the middle of the first century CE, a time in Roman history when violence and intrigue were the order of the day. Seneca had lived through the assassination of the Emperor Tiberius, in which the subsequent emperor Caligula may have participated. He had lived through the assassination of Caligula by Roman soldiers, who then compelled Claudius to assume the principate. Around the time that the Thyestes was composed, Claudius was poisoned to death by his own wife, Agrippina, the mother of Nero.
Given the climate in which Seneca lived, it is perhaps no surprise that the playwright found it appropriate to take up this story of two brothers, Atreus and Thyestes, at odds with one another over the kingship of Mycenae. Thyestes had stolen the kingship from Atreus by, among other things, seducing Atreus' wife. Later, though, when Thyestes had to give up the kingship, Atreus exiled Thyestes. Atreus decides he wants further vengeance on his brother, so he decides to summon Thyestes to the palace under the pretense of sharing the kingdom. When Thyestes returns, Atreus planned to hold a banquet at which he would kill Thyestes' children and manage to have Thyestes eat his own children.
When Thyestes and his sons come to Atreus' home, Thyestes falls into Atreus' trap. He dines with his brother, who has killed Thyestes' children and intermingled their flesh with the food. After dinner, Thyestes initially thinks that he has regained his good fortune. When he asks his brother where his children are, Atreus displays their heads and reveals the truth about the contents of the feast. Thyestes reacts with horror:
What words will suffice for me? I see the severed heads, the torn-off hands, the feet wrenched from the broken legs – this much the father, for all his greed, could not devour. Their flesh is turning round within me, and my imprisoned crime struggles vainly to come forth and seeks way of escape. Give me thy sword, O brother, the sword reeking with my blood; by the steel let deliverance be given to my sons. (F.J. Miller translation).
The play ends with Thyestes' praying for the gods to take vengeance upon his brother.