The Argo II flies over mountains in Italy on a course from Rome to the House of Hades in Greece, where they hope to find Percy and Annabeth, who fell into Tartarus at the end of The Mark of Athena. Hazel, Nico, and Leo guard the ship against boulders thrown by the numina, mountain gods and sons of Gaea who are trying to stop their quest, while Jason, Piper, Frank, and Coach Hedge rest. The ship sustains damage, so Leo retreats out of the mountains. Leo wants to wake the others to discuss a plan to cross Italy, but Hazel knows they must figure it out without them because the group has been arguing since losing their leaders, Percy and Annabeth. Hazel asks for help from her father Pluto, and her magical horse Arion appears.

Hazel is amazed Arion came to help her because the Mediterranean is dangerous for demigods and their allies. She summons Arion a gold nugget to eat because she can extract precious metal from the earth. Leo is concerned because Arion wants to take Hazel to a Roman temple in a tornado. Hazel knows she must go because this is her chance to lead and save her friends, so she lets Arion take her.