Act 1
Percy Jackson, a 12-year-old boy with ADHD and dyslexia, is on a field trip to the New York Metropolitan Museum of Art. While there, his substitute pre-algebra teacher, Mrs. Dodds, asks to speak with him. Once alone, Mrs. Dodds transforms into a Fury, a mythological Greek demon of Hades. Thanks to a pen that transforms into a sword named Riptide, thrown to Percy by his Latin teacher, Mr. Brunner, Percy manages to fend off and vaporize Mrs. Dodds. After this incident, Percy is expelled from his school due to him failing to stay with the group, and him already being on probation. When Percy tries to explain what happened, he is shocked to find that neither Mr. Brunner, his best friend Grover Underwood, nor anyone else remembers Mrs. Dodds ("Prologue/The Day I Got Expelled"). Percy says goodbye to them and heads home for summer vacation.
Back at his apartment, his mother, Sally Jackson, seems to understand and even forgive Percy's expulsion, while her husband, Gabe Ugliano, does not. “Smelly” Gabe abuses Percy, and Sally confesses she needs to tell him about his father, who left before Percy was born. Percy, extremely bitter about his father and upset with himself for being expelled yet again, laments on his actions, being increasingly hard upon himself. Sally assures Percy the quirks and abnormalities of someone is what makes them special ("Strong"). Sally takes Percy to the beach where she met Percy's father, and the two run into Grover. Shockingly for Percy, Grover turns out to actually be a satyr, a Greek goat-like protector.
Suddenly, a Minotaur (half-bull, half-man) attacks the trio. Sally sacrifices herself so that Percy and Grover can make it to a place she calls "camp". The Minotaur kills Sally right before Percy's eyes who, in vengeance of his mother, finishes him off. Percy is kicked in the head and falls unconscious, where he dreams of a man in a Hawaiian shirt, who gives him a seashell, claiming, "What belongs to the sea can always return to the sea" ("The Minotaur/The Weirdest Dream").
When Percy awakes, he finds himself in a place called Camp Half-Blood. The shell the strange man in his dream gifted him remains in his pocket. The camp director, Mr. D (who is actually Dionysus, god of wine and madness), reluctantly explains to Percy that he is a demigod, the son of a human and a Greek god ("Another Terrible Day"). Mr. Brunner (who is really Chiron, an immortal centaur), is also at the camp and explains to Percy that the gods will send a sign to claim him. However, Percy is still skeptical and angry that his father has shown no sign of care for him all these years. Luke Castellan, a nineteen-year-old son of Hermes, sympathizes with Percy, telling him that many half-bloods never know their godly parents, as they are not claimed ("Their Sign").
Percy settles in and meets several other campers such as Silena Beauregard (a daughter of Aphrodite), Katie Gardner (a daughter of Demeter), Clarisse La Rue (a daughter of Ares who takes an immediate disliking to Percy), and Annabeth Chase (a daughter of Athena who took care of Percy while he was unconscious). Annabeth quickly takes the helm of the leader during a game of capture the flag, instructing Percy to sit and wait in the boys' bathroom to assure he won't "mess things up". Clarisse singles him out and tries to "pulverize" him, but the toilets unexpectedly burst to life, dousing her ("Put You in Your Place"). After things settle down, the campers, bar Clarisse, join around the campfire and vent about their unsteady, and in some cases harmful, relationships with both their godly and mortal parents ("Campfire Song").
Percy is claimed as the son of Poseidon, god of the sea, and it is at once rumored that Zeus' lightning bolt has been stolen and Percy is the number one suspect. Percy is told he and two others must go on a quest to retrieve the bolt, to prevent a war between the gods. He is also sent to receive a prophecy from the Oracle of Delphi ("The Oracle"). After the Oracle's prophecy, Percy is upset that he has to leave Camp Half-Blood for a quest that will be unsuccessful, and in turn, externalizes his grievances ("Good Kid"). Percy however, only accepts the quest because Luke hints that his mom will be in the Underworld, the target location of the search since Hades is the rumored real thief. Annabeth and Grover force themselves into the quest, and after a pair of winged shoes is given to them by Luke, the three are pushed into the woods with little care and protection ("Killer Quest!").
Act 2
The three questers have just escaped a bus, attacked by the three Furies. Just after blowing it up, they become hopelessly lost ("Lost!"). In a frantic decision, Percy suggests they should enter Aunty Em's Garden Gnome Emporium. The strange Auntie Em requests to take pictures of the three, but is revealed to actually be Medusa. Percy cuts off her head with his pen/sword, Riptide, and, as a joke, sends the head to the gods over the mail. Annabeth seems particularly upset with this run-in, so while Grover scouts out their surroundings, Percy confronts her about this. She confesses that for her whole life she has been ignored by everyone around her, and is desperate for a chance to prove herself to Athena, her mother, and make the history books ("My Grand Plan").
Grover returns with train tickets to St. Louis and they set off, encountering many dangers such as a Chimera, nasty storms, some dam snacks, and the Lotus Hotel and Casino. They also meet up with Ares, who gives them a lift to Nevada ("Drive"). While on a bus ride to Los Angeles, Percy has a dream of a man speaking with someone whom the man refers to as "my lord". The powerful voice mentions sacrifices and brings up a name, Thalia ("The Weirdest Dream Reprise"). Percy wakes with a jolt and asks Grover if he has ever heard of someone named Thalia. Grover confides that a few years back he was sent to escort Luke, Annabeth and Thalia Grace, Zeus's daughter, to camp. However, the team was attacked, and Grover failed to save Thalia, who ended up sacrificing herself, and being turned into the tree that protects the borders of Camp Half-Blood. He holds himself responsible and thinks Percy will be ashamed of him ("The Tree on the Hill"). Percy assures Grover that no matter who he is or what he does, Percy will always want him as a friend.
The trio arrive in the Underworld, where they are given a tour of the horror by Charon and several long-dead musicians ("DOA"). Percy realizes that the lightning bolt is hidden in his backpack, and the three work out the possibilities of how it could have gotten there. An incident where Percy is nearly dragged into Tartarus by the shoes Luke gave him occurs and the group is discovered. After a brief conversation with an innocent Hades, Percy withdraws the shell from his pocket and blows it, realizing it was from Poseidon, and it opens a portal out of the Underworld. Vowing to return to save his mom, Percy, Annabeth, and Grover escape. Percy hypothesizes that Ares is the one who planted the bolt in his bag and comes to terms with both himself and Poseidon, declaring himself a son of Poseidon, and uses his gift with water and the help of Annabeth and Grover to best Ares ("Son of Poseidon").
The trio return to camp heroes, but Percy couldn't be more troubled. He confides in Luke that he feels just as confused as before, and Luke agrees, stating he felt similar after his own quest. Luke inadvertently tells Percy that he is the true lightning thief and that he has teamed up with Kronos to get back at the gods, who he feels have done him wrong ("The Last Day of Summer"). Luke attacks Percy and escapes. Percy declares that there will be a war no matter how they try to stall it, but he and his friends will be prepared and do whatever it takes to fight it ("Bring on the Monsters").
