In the beginning of the book it starts off in the Argo II the ship leo built so that they could find the roman camp. This is the next place it is set at the roman camp where they try and fulfill the prophecy of the big 7. After this they sail off to Rome to do their quest.
Annabeth, Leo, frank, Jason, piper and coach hedge take the Argo II to get Percy back from the roman camp. When at the roman camp an eidolon takes over Leo and makes him shoot his ballistae at the roman camp which aggravate the romans and they try and fight the Greeks. But the Greeks are just trying to make peace so they can come together and defeat Gaea. An eidolon is an evil spirit that Gaia sent to take over Leo’s body and try and destroy the roman camp. But the big 7 find their way to the ship and decide to go on the quest to Rome so the romans can’t follow them. On their voyage they stop to try and find Bacchus to help with their quest. When they go to Kansas where they think he is piper Jason and percy do not find him and end up fighting each other instead. The eidolons take over their bodies and they start fighting but piper has charmspeak a power she gets for being a daughter of Aphrodite. She makes the eidolons swear on the river Styx that they will never possess any of her friends ever again. Next they go to look for a map in Charleston, south Carolina which is crucial in annabeths quest to find the mark of Athena. The romans find the 7 demigods in Charleston and begin to fight them Annabeth and Reyna meet and Annabeth tells Reyna if they let them go she can reunite the romans and Greeks. So they retreat and let them go on their voyage to Rome. On their way to rome they cross paths with another son of Poseidon on the Mediterranean. Chryasoar is his name he takes the ship over with his half dolphin half men but the 7 demigods trick his men and chryasoar into exploding his own ship and they get away. When they reach rome they all split up to find different parts of their quest. Annabeth goes off on her own to find the Athena parthenos following the mark of athena. This statue could make or break the outcome of the world as they know it. Leo hazel and frank go off to find nico in a museum where he is supposedly being held by Gaea’s twin giants Otis and ephialtes. There they find a secret tunnel but find out it is a dead end. Piper Jason and Percy also go to find nico where they run in to nymphs that get ignored and try to drown them in their poison but end up saving the nymphs and themselves...