Percy Jackson is having a decent time on his school field trip until his math teacher turns into a bat-winged hag and tries to kill him. Wheelchair-bound Mr. Brunner comes to his rescue and throws him . . . a pen. Much to Percy’s astonishment, the pen turns into a sword. One slash, and the hag turns to dust.

The weirdest thing? Mr. Brunner pretends it didn’t happen. Percy’s best friend, Grover Underwood, plays along. But Percy isn’t fooled. Mr. Brunner and Grover are hiding something from him, he’s sure of it.

Turns out he’s right. One stormy night, he’s with his mother in a seaside cabin in Montauk, New York, when Grover shows up with some seriously bad news: A monster is coming. If he doesn’t come with Grover right now, Percy will be killed. On an unrelated note, Grover reveals that he isn’t a teenage boy. He’s a satyr: half man, half goat.

These revelations pop the cork on a big bottle of “What now?” But explanations will have to wait. The threesome flee by car, pursued by a minotaur. Correction: <em>the</em> Minotaur, the original monster from Greek mythology. It’s big, it’s hairy, and it’s gaining.

They’re nearly at the camp’s hillside border when—boom!—an explosion rocks the car. Grover is knocked senseless. As Percy and his mother drag the satyr free, the Minotaur appears and charges. Percy evades the attack. His mother isn’t quick. Before Percy can react, the monster grabs her by the neck and squeezes. She vanishes in a flash of light.

Enraged, Percy races up the beast’s back and snaps off one of its horns. Seconds later, the Minotaur dissolves into dust. Percy drags Grover to a pine tree on top of the hill and collapses.

When he comes to, he’s in the infirmary of the Big House, Camp Half-Blood’s headquarters. But what exactly is Camp Half-Blood? Percy could get the answer to that question from the camp’s orientation film, but there’s a better source—his old teacher, Mr. Brunner, whose real name is Chiron. And like Grover, Chiron has a secret. Hidden within his wheelchair is the body of a horse. Chiron is a centaur.

A satyr, a centaur, the Minotaur . . . Percy’s mind is blown, and the revelations don’t stop there. Chiron explains that the Greek gods of mythology are real. Now and then, they form relationships with special humans, just as they did in ancient times. Sometimes those relationships result in children. Demigods, those children are called, or half-bloods.

Percy makes the connection. Camp Half-Blood is a camp for demigods. Which means . . . he’s a demigod, the son of a god. But which god? He finds out when a hellhound bounds out of the woods and attacks him during a violent game of capture the flag. He should be dead, but when he steps into a nearby creek, energy surges through his body. His wounds heal. Moments later, a glowing trident symbol appears over his head. The sea god Poseidon has claimed him.

That means big trouble for Percy. Not long ago, Poseidon quarreled with Zeus. Afterward, Zeus discovered that his master bolt, the king daddy of all lightning bolts and his most important weapon, was missing. Poseidon couldn’t have taken it himself. But he could have had his son steal it. Percy’s life takes a sudden dramatic turn. With Grover and Annabeth Chase, the intimidating daughter of Athena, he sets off on a quest to discover the real thief, recover the master bolt, and clear his name. The harrowing journey finds the heroes trapped in a Las Vegas hotel, nearly stretched to death on a water bed, and venturing deep into the realm of the dead.

It’s all pretty overwhelming, but Percy doesn’t get really scared until he learns who stole the bolt, and why. Luke Castellan, son of Hermes and Annabeth’s dear friend, has betrayed them to join with the Titan Kronos, the ancient lord of time who was overthrown by the Olympians millennia ago. With Luke’s help, Kronos is emerging from Tartarus. If he succeeds, it will mean the end of the Greek gods—and their children.