When Agamemnon arrives home after fighting in the Trojan War, he brings with him Cassandra, the daughter of King Priam, who Agamemnon has now taken on as a concubine. 

Despite the spectacle that Agamemnon is creating by flaunting his mistress in front of her, Clytemnestra greets him with declarations of her fidelity, assuring her husband that she has faithfully awaited his return. She claims that this period of waiting was one of great suffering for her, stating:

Had Agamemnon taken all
the wounds the tale whereof was carried home to me,
he had been cut full of gashes like a fishing net...

In reality, Clytemnestra has begun a torrid love affair with Agamemnon's cousin, Aegisthus, and is intent upon murdering her husband in order to punish him for his long absence and to avenge the death of their daughter, Iphigenia, who Agamemnon had previously sacrificed. This intention eventually comes to fruition when Clytemnestra convinces Agamemnon to walk across a purple tapestry (a symbol of hubris) into a bath, where she murders him with a double-bitted axe.

In section 598 (of an e-version of Agamemnon), Clytemnestra is speaking to the Chorus of Argive Elders. It is here that she states her loyalty to her husband, Agamemnon. While speaking with the elders and herald, and asking the herald to take a message to her husband, she states that she has been utterly faithful, "a watchdog of his house," one who would never wish ill upon him, and one who would never break any seal (promise).

Unfortunately, Clytemnestra's words are not truthful. Although she claims loyalty to her husband, the herald seems to question the words she speaks. The chorus, then, states that truth lies in interpretation. Clytemnestra, in fact, has not been loyal. Instead, while her husband was fighting the Trojans, she had an affair with Aegisthus. Even more despicable is the fact that she is planning the murder of her husband.