At the end of Agamemnon, Cassandra and Agamemnon are lying dead and unburied; the Chorus has just narrowly avoided fighting Aegisthus and has called for the return of Orestes to avenge his father; and Clytemnestra has announced her intention to share rulership of Argos with her lover, Aegisthus. This leaves a lot of loose ends to tie up, doesn't it?
Of course it does! That's because Agamemnon is only part one of a three-part trilogy of tragedies called the Oresteia (the other two are Libation Bearers and Eumenides); originally, all three tragedies would have been performed on the same day at the festival of Dionysus in Athens. It wouldn't have been very smart of Aeschylus to wrap everything up nicely at the end of the first play. If he did, people would probably have wandered off in search of lunch or the ancient equivalent of cotton candy, or something (provided that the gruesome murders at the end of Agamemnon hadn't made them lose their appetites). So, it's best to think of the end of Aeschylus's Agamemnon as a cliffhanger.