After choosing the left tunnel, Percy, Annabeth, Grover, and Tyson hit a dead end blocked by a huge boulder while something inhuman pursues them. Tyson forces the boulder aside, and the group slips through, only to discover they have entered a barred cell in a large prison. Annabeth recognizes the place as Alcatraz, meaning the Labyrinth has carried them from Camp Half-Blood to San Francisco.

Tyson hears an ancient language and rushes ahead despite Grover's warning. The group hides and sees Kampê, a terrifying dragon-bodied jailer with snakes and shifting monster heads, threatening a prisoner named Briares. Tyson translates that Kampê is trying to force Briares to serve the master, and Annabeth explains that Kampê once imprisoned the Cyclopes and the Hundred-handed Ones for Kronos before Zeus freed them.

The group approaches Briares, a pale, human-sized Hundred-handed One with many arms and changeable faces. Tyson reveres him as a legendary, mountain-breaking elder, but Briares is broken by fear and refuses to leave because Kampê has returned. When Tyson realizes Briares will not free himself, Tyson tears off the cell door, and Percy tricks Briares into leaving by winning a rigged game of rock, paper, scissors with a finger-gun.

Kampê reappears, and the group flees through Alcatraz. Outside, Percy notices ordinary tourists and a supernatural storm over Mount Tamalpais, where Mount Othrys is rising. Kampê smashes through the prison wall with poisoned bronze scimitars, and even Annabeth decides the monster is too dangerous to fight.

At the wharf, Tyson chooses to create a diversion so the others can drag Briares back toward the prison and the Labyrinth entrance. Tyson charges Kampê with a lamppost, then hurls an ice-cream stand at her, buying time but placing himself in danger. Annabeth, Grover, Percy, and Briares find the hidden Daedalus mark, and the wall begins to open.

As Kampê gains on Tyson, Percy throws his bronze shield into the monster's face, slowing her just long enough for Tyson to dive into the passage. The stone door seals before Kampê can follow, and the group escapes into the Labyrinth, relieved to be away from Alcatraz despite the dangers still ahead.