Rick Riordan had been working on The Mark of Athena since the completion of The Son of Neptune.
During his tour to promote the last of The Kane Chronicles books, titled The Serpent's Shadow, Riordan read part of the first chapter of The Mark of Athena. The cover and full first chapter were also released Disney's The Heroes of Olympus website. On September 27, 2012, Riordan confirmed that there would be a first US printing of 3.5 million copies.
Six months after the events of The Lost Hero, Leo Valdez has constructed a trireme, Argo II, for use in the seven demigods journey to Greece to stop Gaea from awakening. He, Jason Grace, Piper McLean, and Annabeth Chase, accompanied by Coach Hedge, arrive at Camp Jupiter to rendezvous with Percy Jackson and the Roman demigods. At Percy's request, Frank Zhang and Hazel Levesque are chosen as representatives of the Romans, alongside Jason, to accompany the Greek demigods. Annabeth, while having a talk with Camp Jupiter's praetor, Reyna, about the quest, reveals that in order to unite the Greek and Roman demigods against Gaea, they have to first retrieve the Athena Parthenos, a giant statue of Athena that was stolen by the Romans in ancient times and hidden somewhere in Rome. The statue can only be retrieved by the demigods of Athena through the help of the Mark of Athena, all of whom had died trying. However, an Eidolon sent by Gaea possesses Leo and forces him to fire a blast on the camp, inciting the paranoid augur, Octavian, to convince the Romans that the Greeks are a threat and have to be destroyed.
The seven demigods escape from Camp Jupiter and briefly land near the Great Salt Lake to search for celestial bronze, which they steal from a resurrected Narcissus. During the search, Leo meets with Nemesis, who presents him with a fortune cookie that would help him later, but with consequences. Percy, Jason, and Piper meet with Bacchus at Kansas after hearing about his canceled appointment with Ceres. With Gaea's directing, two Eidolons possess Percy and Jason, and later Leo, but both times they are repelled. Percy, Frank, and Hedge also face Phorcys and his sister, Keto at the Georgia Aquarium while searching for information about Gaea's plans, with the latter sending one of her children, a giant Scolopendra, after them. Throughout the journey, tensions arise between the demigods, mostly between Leo and Frank, the latter of whom constantly becomes the target of jokes and teases by the former and is jealous of the former's friendship with his girlfriend, Hazel, when he learns that Hazel's previous boyfriend is Leo's identical great-grandfather. Hazel is also upset with Jason and Leo because the two are hesitant in saving her half-brother, Nico di Angelo, who has been captured by the Giants during his travel to find the Doors of Death. While searching for the Mark of Athena at Fort Sumter in Charleston, the demigods are ambushed by the Romans, but Reyna decides to let Annabeth continue her search for the Athena Parthenos, reminding that their next encounter will no longer be easy.
While crossing the Atlantic Ocean, Leo, Frank, and Hazel are sent underwater when the Argo II is attacked by the Scolopendra, in the process learning about the existence of Camp Fish-Blood, a hero-training camp for ichthyocentaurs. The demigods resume the travel after regrouping and arrive at Europe. After managing to get around from Hercules at the Strait of Gibraltar, they sail through the Mediterranean Sea, confronting Chrysaor along the way, and finally reach Rome. There, the group split up, with Hedge defending the ship, Percy, Jason, and Piper scouting the Colosseum, Frank, Hazel, and Leo searching for Nico, and Annabeth finding the Athena Parthenos. Percy and Jason are confronted by Ephialtes and Otis, the twin Giants who capture Nico, and have to incapacitate them a few times before Bacchus arrives to finish them off and claim the victory. Frank, Hazel, and Leo are trapped by the Eidolons underground and Leo decides to use his fortune cookie, which suggests him to make use of the Archimedes Ball found there to bail them out and seal the Eidolons for good. Annabeth, meanwhile, has to cross a variety of challenges designed to trap the demigods of Athena, eventually confronting Arachne, the last guardian of the statue and a seamstress who was turned by Athena to a spider for boasting about herself. Through tricks and deception, Annabeth manages to bind Arachne inside her own trap, and the other demigods use Argo II to push Arachne down towards Tartarus, located immediately below Arachne's lair. The demigods secure the Athena Parthenos and save Nico; however, Arachne uses her remaining silk to stick Annabeth's feet and pull her down with her, and when Percy attempts to save her, he is pulled down too. Before they both fall, Percy asks Nico to meet them at the other side of the Doors of Death.
While others are uncertain about Percy and Annabeth's fate, Hazel and Nico, being the children of the god of the dead, assure them that the two are not yet dead. Leo, who realizes that Percy and Annabeth's fall are the "consequences" mentioned by Nemesis, feels personally responsible and commands Argo II to set sail towards Greece.Annabeth Chase – Daughter of Athena (called Minerva by the Romans) tasked with recovering the Athena Parthenos. Girlfriend of Percy Jackson.
Leo Valdez – Son of Hephaestus (known as Vulcan in the Roman pantheon)
Piper McLean – Daughter of Aphrodite (known as Venus by the Romans) girlfriend of Jason Grace.
Perseus (Percy) Jackson – Son of Poseidon (called Neptune by the Romans) who recently recovered from amnesia Also formerly praetor in Camp Jupiter. Boyfriend of Annabeth Chase.
Jason Grace – Son of Jupiter and praetor of Camp Jupiter, recently recovered from amnesia. Boyfriend of Piper McLean.
Frank Zhang – Son of Mars and legacy of Poseidon. Boyfriend of Hazel Levesque.
Hazel Levesque – Daughter of Pluto, recently returned from the dead. Girlfriend of Frank Zhang.
The Aloadae: Ephialtes and Otis – These twin giants were born to oppose Dionysus/Bacchus, and as such, are natural showmen.
Arachne – A prideful weaver turned into a spider for boasting she was better than the goddess Athena.
Arion – Immortal horse son of Ceres and Neptune; Hazel's mount.
Athena/Minerva – Goddess of strategy, intellect, and crafts.
Ella – Harpy with a love for books. She has memorized the Sibylline Books.
Gaea – Primordial goddess of the earth who wishes to overthrow the gods so her older children, the giants, may rule.
Coach Gleeson Hedge – Satyr Keeper of Camp Half-Blood. He is the chaperone on board the Argo II.
Nico di Angelo – Son of Hades, captured by Ephialtes and Otis.
Octavian – Legacy of Apollo and power-hungry augur of Camp Jupiter.
Reyna Ramírez-Arellano – Daughter of Bellona and praetor of Camp Jupiter.
The Mark of Athena was released on October 2, 2012. It is available in hardcover, paperback, eBook and audiobook formats. The following book of the series, The House of Hades, was released on October 8, 2013.
Meann Ortiz of GMA Network gave the book a mixed review, saying that it was interesting to watch the characters grow up in another installment of the series and praised the cliffhanger ending but that the romantic subplots felt forced: "'Girlfriend' and 'boyfriend' are bandied around more times than necessary to establish who is with whom, that I feel like Riordan drops a Relationship Anvil on my head every time I see those words." Overall, Ortiz found the multiple perspective format tiresome and noted that the plot was very similar to that of previous books, saying it was a "testament to Riordan’s skills that the books never end up being too boring and predictable." A reviewer from The Guardian was less troubled, calling this "the best in the series so far" and praising its "funny bits" while noting that the perspective format was lopsided in favor of Annabeth, Percy and Jason.
During its first week, The Mark of Athena sold about 237,000 copies.