Or rather, one boy in particular – Orestes. As we find out a bit later, Orestes has been promising to return to Mycenae for ages, and it's only now that he's gotten around to it. Scholar Pat Easterling refers to this as "zero hour," and points out that many of Sophocles plays begin this same way.
The central conflict is that Clytemnestra and Aegisthus are murderers, and have yet to be punished for their deeds. But Chrysothemis raises the conflict stakes when she reveals to her sister that Aegisthus has plans for her that involve a slow and painful death inside an abandoned cave, presumably where no one can her scream.
We're going to talk about complication on two different levels:
This is the moment the play has been building towards – revenge against the woman who murdered her husband. For the main character, Electra, this is an emotional and decisive climax. Electra is fully committed to the action, as we gather from her urging Orestes to strike the Queen a second blow.
This scene is edge-of-your-seat action because we know something that Aegisthus doesn't: Clytemnestra's dead body is under the sheet. We're tingling with anticipation while Orestes slowly reveals his identity.
Certainly not the heart-thumping action of Clytemnestra's death. This death is anti-climactic, a forgone conclusion.
Most translations render the Chorus's final stanza congratulatory, a sort of, "Yes! Electra, you did it!" kind of thing. But David Raeburn (translator of the Penguin Classics edition) nods to the play's moral ambiguity in his translation, suggesting that Electra's feud has come to an end, but so has her moral character. See "What's Up With the Ending?" for more juicy details.