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.
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.