In this third book in the series, Thalia, daughter of Zeus, brought back to health by the Golden Fleece (her essence was put into a tree after she saved Luke and Annabeth, two other half-bloods, battling monsters getting to Camp Half-Blood), is the latest complication in Percy’s life. A prophecy says that a son or daughter of one of the big three (Zeus, Poseidon, Hades) could bring down Olympus on their 16th birthday. With Thalia back on the scene, either she or Percy (son of Poseidon) could fulfill that destiny.
With the battle with the Titans looming, the search for allies and more heroes expands. Grover (the satyr) finds two half-bloods, Nico and Bianca di Angelo, at a school, being stalked by a monster. Percy, Annabeth and Thalia launch a rescue, and are then themselves rescued by the goddess Artemis and her Hunters. During this final battle, Annabeth is apparently killed by the monster they were fighting, and the goddess Artemis goes hunting one of the monsters (as she is the goddess of the hunt) and is also lost.
As with every book in this series, a hero is granted a quest. The quest is to save Artemis, who apparently (according to hero’s dreams) saved Annabeth by taking her place holding something up (when Annabeth saved Luke, a half-blood who now serves the Titan Kronos). Artemis’ right-hand huntress Zoe is given the quest, with Bianca, Thalia, Percy and one other huntress joining the quest. Bianca joins the Hunters instead of remaining a half-blood, much to the chagrin of the heroes (and her brother).
Also with each book, Percy meets a new god and a new Titan. Apollo (Artemis’ brother) and Atlas (who holds up the sky from the earth) make appearances in this novel. As with the previous novels, Riordan intertwines excellent knowledge and mythology of the gods of Olympus and the Titans with great plot pacing and an excellent sense of humor. In addition, the plot is unpredictable, given the plethora of possible gods and heroes at Riordan’s disposal. He is obviously marching to a showdown between the gods and the Titans.
The ending (which won’t be spoiled here) again very nicely sets up the next book (or books).