The novel begins with Percy Jackson being pursued by gorgons. Percy has had a rather bad week - he can't remember anything about his past except for his name and a girl named Annabeth. He is being hounded by monsters from mythology, and he has to get Camp Jupiter for safety. When he finally arrives at Camp Jupiter, he still doesn't know if he's actually safe since he is met with suspicion. His abilities clearly mark him as a son of Neptune, who isn't exactly a favorite among the Romans. He quickly makes friends with two of the other misfits in the camp - Hazel, a daughter of Pluto, and Frank, who is still unclaimed though his 16th birthday has passed. Hazel has secrets about her past that she is hiding, and Frank is still grieving for his mother, a Canadian soldier that died in Afghanistan.
Percy's first day at the camp is very eventful, and ends with an appearance from Mars himself sending Frank, Percy and Hazel on a quest. It was actually a very funny scene, especially when one of the Roman teens insists that quests come from scrolls and prophecies until Mars finally humors him and writes it out on the spot. The problem in this novel is that Gaea and her forces have captured and imprisoned Thanatos, or Death, which is why all their slain enemies keep returning. The trio must go to Alaska to save Death and restore order.
The previous novel in this series dealt with a Roman hero suffering from memory loss in a Greek camp, and this one does the opposite. While Riordan already used Jason to begin alluding to differences between Greeks and Romans, actually being in the Roman environment really made it hit home. The camp revolves around the cohort and the army. The campers are organized into cohorts regardless of who their immortal parents may be in contrast to the Greeks who are housed based on parentage. The Romans are also more focused on discipline and duty.
Percy's memories come to him much more quickly over the course of the novel than they did to Jason, but that is also based on Hera/Juno's timing of their adventures. Like all of Riordan's novels, this one is another engaging adventure with sweet characters who sometimes think they have dark secrets but learn to trust each other, and a nice mix of the modern and ancient.