What are the suitors' plans for Telemachus? How does Penelope hear of the plans?
Penelope's suitors plan to kill Telemachus as he returns from his voyage to find Odysseus. She finds out about the plot when Medon, a herald, overhears their plans and comes to alert her about the danger to her son.
When Penelope's suitors realize Telemachus has gone to find his father, they realize that they can use his voyage to kill him. If they kill him while he is on his way home, it will not appear that they were the ones who did it. Instead, it will look like he died on his journey. Antinous says:
Zeus kill that brazen boy before he hits his prime!
Quick, fetch me a swift ship and twenty men—
I’ll waylay him from ambush, board him coming back
in the straits between Ithaca and rocky Samos.
This gallant voyage of his to find his father
will find him wrecked at last!
Luckily, Medon is listening without the suitors being aware. He is loyal to Odysseus and Penelope, so he goes to tell her what he has heard.
sure of his own discretion, “Ah my queen,
if only that were the worst of all you face.
Now your suitors are plotting something worse,
harsher, cruder. God forbid they bring it off!
They’re poised to cut Telemachus down with bronze swords
on his way back home. He’s sailed off, you see . . . for news of his father—to sacred Pylos first,
then out to the sunny hills of Lacedaemon.”
When she hears the news, Penelope feels panic. She was not aware that Telemachus had left to find his father. She rails at her female servants for not alerting her to his voyage. Later, she confronts the suitors. The only response she gets is Eurymachus telling her to be calm and not worry because there is no man "who'll lift a hand against Telemachus, your son" as long as he, Eurymachus, is alive to defend him. He says that his loyalty is to Odysseus.
Of course, Eurymachus is wrong. Eumaeus later warns Telemachus that from Hermes's Ridge he "caught sight of a trim ship pulling into the harbor, loaded down with a crowd aboard her, shields and two-edged spears. I think they're the men you're after—I'm not sure." The suitors don't know that Telemachus is already past their grasp.
In Homer's Odyssey, Odysseus' son Telemachus decides to go out in search of his long-lost father. At the end of Odyssey 2, Telemachus sets sail from Ithaca. In Odyssey 3, Telemachus travels to visit King Nestor in Pylos, and in Odyssey 4, Telemachus questions King Menelaus and Queen Helen in Sparta.
Near the end of Odyssey 4, Penelope's suitors plot to ambush Telemachus upon his return and kill him (lines 625-674). The suitor Antinous declares that "I’ll lie in wait for him in the straits as he makes his solitary passage between Ithaca and rocky Samos, and his voyage in search of his father will end sadly" (Kline translation).
Immediately after the plot is made, Medon, who serves as the herald of the suitors, goes to Penelope and reports the plot to her (Odyssey 4.675-720).
Homer keeps the audience in suspense about this plot, though, and we do not learn what happens until Odyssey 15.1-55, where Athene visits Telemachus and warns him about the suitors' plot. Thus, thanks to the help of the goddess, Telemachus avoids the suitors' trap.
While Telemachus is in Sparta speaking with Menelaus and Helen and asking them for advice or news of his father, the suitors back in Ithaca find out about his secret departure. Angry, the suitors plan an ambush when he returns home. They plan to kill Telemachus before he reaches home, and this will open up the power of the new king (assuming Odysseus does not ever return).
Medon, who is a herald in Ithaca, overhears what the suitors are planning, and he rushes to inform Penelope of their evil plan. She did not know of Telemachus' trip, either at this point.