The Battle of Montauk was a battle between Percy Jackson, Carter Kane, and a petsuchos.
After escaping with the Book of Thoth, Setne began experimenting with combining Greek and Egyptian magic. As part of this, apparently to see what would happen when the two meet, Setne put a petsuchos necklace on a baby crocodile and turned it into a petsuchos, a giant crocodile and a son of the crocodile god Sobek. He then released it in Long Island where he knew it would draw the attention of Camp Half-Blood and Brooklyn House.
After the House of Life scryers began noticing magical disturbances in Long Island and there were news reports of what the locals called the Long Island Swamp Monster which was eating wildlife and scaring the locals near the Montauk Highway, Carter Kane decided to check it out, going on his own since the initiates of Brooklyn House and his sister were busy at the First Nome learning how to control cheese demons. Unknown to him, the petsuchos, which had been loose for weeks, also drew the attention of Percy Jackson when it ate a pegasi. Arriving in a Montauk swamp on his griffin Freak, Carter found the petsuchos which swallowed him whole. As Carter got ready to use the Fist of Horus to try to free himself, Percy attacked the monster, stabbing it in the rump with Riptide and causing it to spit out Carter and flee.
Upon meeting, the demigod and magician were confused by each other and battled to a draw before calming down and agreeing to work together to stop the petsuchos. Carter explained what it was, having figured out the type of monster when he saw its necklace and that they couldn't kill it as they headed to where the petsuchos was attacking a nearby neighborhood. As they arrived, they found it destroying a Prius and under a useless attack by kids with water balloons and Super Soakers.
Seeing that the kids were in danger attacking the petsuchos, Carter yelled at them to run away while throwing his wand at the monster's head and uttering the Divine Word Sa-mir or pain, causing the hieroglyph for it to appear all over the petsuchos, causing it to smoke and spark and causing it a lot of pain. Seeing that, the kids hid behind cars and ruined mailboxes and stopped attacking the petsuchos which turned its gaze on Carter. After confirming that the petsuchos can't be killed, Carter tells Percy that the only way to defeat it is to remove the necklace which will cause it to turn back into a regular crocodile. Percy decides that Carter will distract it while he gets the necklace and moves away as the monster charges Carter. As the kids resume their attack on the petsuchos, Carter summons his combat avatar which shocks Percy so much he stops in his attempts to reach the necklace. As the petsuchos grabs him by his free arm, Carter tries to slash the necklace off with his khopesh but instead slashes the monster in the shoulder. However, it doesn't seriously injure the monster and the wound heals as the sword is removed. The petsuchos then shakes Carter from side to side and sends him flying through the roof of a house. After seeing that Percy is too shocked to continue with his part of their plan, Carter reminds him and Percy goes after the monster as it charges Carter again. Carter kicks the petsuchos in the snout, distracting it long enough for him to get up as Percy jumped on its back and ran up its spine and the mortal kids continued their attack with rocks, pieces of scrap metal from destroyed cars and tire irons. To keep the petsuchos from going after the kids, Carter tries to slash its snout with his khopesh, but the monster is able to grab the sword and wrestles Carter for it, causing its teeth to crumble to sand while Percy manages to reach the necklace and tries to break it with Riptide to no avail. Percy eventually realizes he can't cut it with Riptide and tells Carter who spots the clasp and tells him to open that instead. As Percy climbs to the clasp, Carter starts to run out of energy and loses his combat avatar. He only survives losing it because when the avatar's sword that the monster had been pulling at disappears, it causes it to lurch back and trip over a Honda. The petsuchos quickly recovers and has a stare-down with Carter while Percy determines that he can't unlock the clasp which Carter realizes is because only a magician could figure it out. Carter quickly starts to make a shabti, the last thing he can think of to do and when the petsuchos charges again, throws the shabti at it and activates it. The shabti takes the form of a misshapen hippopotamus and flies into the petsuchos left nostril, causing it a lot of pain and distracting it as the petsuchos tries to free itself. Unable to unlock the clasp, Percy takes the opportunity to jump down and join Carter, pulling him out of the petsuchos path just in time as it started to trample around. Trying to get rid of the shabti, the thrashing of the petsuchos destroys several homes and Percy sends the kids who were trying to fight it to stall the approaching police. Carter informs him he's out of tricks but thinks he can open the clasp if he can reach it. Against Carter's protests that Percy doesn't even have his sword anymore, Percy decides to distract the monster himself while Carter tries for the clasp and charges the petsuchos as it finally frees the shabti from its nostril and blasts it across Long Island.
To Carter's surprise, Percy stops in front of the petsuchos and uses his power over water to use the water the petsuchos continually sheds to create a whirlpool so strong it causes the petsuchos to struggle with its footing. Realizing he has the distraction he needs and that the flow of the water is too strong for him to run to the petsuchos, Carter uses his last bit of magical energy to turn into a falcon despite worrying that it would cause him to burn up. Carter is left stunned by the power of Percy's whirlpool which is so strong that the petsuchos struggles against it, wrecked cars scrape along the pavement and mailboxes are ripped out of lawns. Realizing that Percy can't control that much power for long, Carter quickly flies to the necklace and turns back into a human and grabs hold as he reaches the clasp. As Percy and his whirlpool starts to weaken, Carter works on figuring out the secret of unlocking the clasp. Carter tries pushing the fourth hieroglyph in Sobek's name on the clasp, but it does nothing and the whirlpool weakens to the point that the petsuchos is able to turn and face Percy who is forced to one knee from his weakening strength. Carter finds that the third hieroglyph seems slightly warm when he touches it, but nothing happens as he tries to push it either. At that moment, Percy's strength gives out and the whirlpool dies so Carter tries hitting the third hieroglyph with his fist and all his strength. It works, the clasp unlocks and the necklace comes off, causing Carter to fall to the ground and be buried by the giant necklace. A moment later, the necklace returns to a normal size and the petsuchos reverts back into its original form of a baby crocodile.
With the battle over, Percy and Carter collect the necklace and the baby crocodile and flee as the kids lead the police to the neighborhood now that the danger is over. Carter and Percy regroup in a diner where they are able to use a healing potion made by Jaz to heal most of their injuries and learn from the news that the battle is being blamed on pressure building up in drainage pipes and causing an explosion that resulted in a flood and eroded the soil so badly that several houses collapsed. Luckily no one was hurt by the battle and no one believed the kids story of two teenagers fighting the Long Island Swamp Monster. Carter and Percy realize that someone put the necklace around the baby crocodile's neck and turned it into the petsuchos and Carter theorizes that while they don't know who did it, the person may have done it to cause trouble that would draw both of their attention, cause them to meet and see what would happen. As a result, the two agree its best not to know too much about each other and don't question each other's abilities. Carter and Percy both agree to investigate the incident separately from their own sides and Percy agrees to let Carter take the necklace to send it somewhere safe. Carter also decides to take the baby crocodile to Brooklyn House to become a new pet there. In case Percy needs his help in the future, Carter paints an Eye of Horus symbol on his palm which, when Percy speaks his name, will let Carter know that Percy needs him and where he is and he can go to him. The two go their separate ways and Carter sends the petsuchos necklace to the First Nome for safe-keeping in their vault. Setne's experiments in mixing Greek and Egyptian magic continue and bring together Carter's sister Sadie and Percy's girlfriend Annabeth Chase to stop the god Serapis. Later, his attempt to become immortal brings all four together to defeat him once and for all by mixing Greek and Egyptian magic themselves, but the four agree that its best to keep their knowledge of each other and team-ups among themselves.