The four demigods are currently on a train. Grover's head resting on Eurydice's shoulder, with her head on top of his as they both sleep. 

Grover groans lifting his head waking both himself and Eurydice up.

"You awake?" Percy asks the two.

Eurydice just nods while Grover snaps, "Well, I am now. Thanks."

"Are you okay?" Percy asks concerned for his friend. 

"He's super grouchy when he doesn't get enough sleep." Annabeth explains.

"'He's super grouchy when he doesn't get enough...' Nah!" Grover mocks and Eurydice stifles a laugh. 

"Wow." Percy laughs. 

"You've never been on the road with him before." Annabeth says, "A little different than a froofy boarding school."

"Who's froofy? You're froofy. What's froofy?" Grover says. "I think I need to eat."

Eurydice nods along with him and the four of them move to get some food from the front of the train.

"Two days until we reach Los Angeles." Grover tells the group as the wait for their food, "Plenty of time before our deadline to reach the Underworld."

"Can I ask a dumb question?" Percy asks the group.

"It's like you need me to make fun of you." Annabeth jokes.

"Shoot," Grover tells Percy.

"I've never been to Los Angeles before." Percy says, "I'm guessing none of you have been to Los Angeles. So, how will we have any idea where we're going?"

"No idea." Eurydice responds honestly, "But that's like step 37, and we're still on step four."

"Cross that bridge when we get there." Grover says.

"Follow up stupid question." Percy says.

"Dude." Grover sighs at his friend. 

"'And you shall fail to save what matters most in the end.'" Percy continues, "Back in Jersey, I told you the Oracle said this quest would fail. And no one's mentioned it since. Seems like something we oughta be taking a little more seriously."

Eurydice's attention get drawn out the window, "Hey. Look at that!"

"Are those..." Percy starts.

"Centaurs," Grover finishes for him. 

"No one even knows they're there." Percy says in awe. 

"There used to be herds of them everywhere." Grover explains.

"What happened to them?" Percy asks.

"Humans." Grover answers, "A few thousand years ago, the god of the wild, Pan, disappeared. And ever since, without Pan to protect the natural world, humans have been trying really hard to chip away at it. The bravest satyrs volunteer to become Searchers, trying to find Pan. None have ever returned."

"Your uncle we found in Medusa's, Ferdinand." Percy says warily, "He was a Searcher?"

"The Oracle didn't say the quest would fail." Annabeth changes the subject. "'Fail to save what matters most.' That could mean a lot of things. That's how prophecies work, that's how fate works. It could mean a lot of things."

"The harder you work to understand, the harder it gets to understand." Eurydice explains, "Sometimes, you've just got to let it come to you when it's ready."

One of the train employees approaches the four and asks to see their tickets. Eurydice hands him the four tickets she held on to for the group. 

He looks at them before raising an eyebrow, "You're in Cabin 17B?" Eurydice nods and he leads the four of them back to their area. They see the window smashed and the whole compartment trashed. 

"Care to explain?" The worker asks. 

"Wait. Wait, you think we did this?" Percy asks back.

"Did you?" The worker presses, "I mean... How? And why?"

"Sir," Annabeth explains, "when we left to get breakfast, everything was intact."

"We don't know how this happened." Eurydice tells the worker.

"We got a witness here, says she heard the window smash, and then heard children's voices." 

"Oh, come on." Percy sighs.

"Can you tell me what time you left the cabin?" The worker interrogates.

"Are we under arrest?" Annabeth challenges.

"I don't think you wanna take that tone with me, little girl."

"Are we under arrest?" Annabeth repeats, not backing down. 

The group then gets taken to a different compartment where they're going to be held until the next stop. 

"So we're just killing time 'til we find out that guy's like a werewolf or something, right?" Percy quips. 

The train PA system announces that the train will be stopping at the St. Louis Gateway station in ten minutes. 

"I don't think he's a monster." Eurydice tells the younger kids.

"Hard to say." Grover says.

"Well, if he's not a monster, what's going on here?" Percy asks. "Why would anyone tear our room apart?"

"Maybe they were looking for something." Annabeth suggests.

"We don't have anything." Percy argues.

"The people who think you stole Zeus's master bolt might disagree." Annabeth replies. 

"Right." Percy sighs. "Well, look, they're not gonna find something we don't have."

"Either way, we aren't spending the day answering questions in the St. Louis police station." Eurydice tells the rest of them, "We need to get out of this before we get delayed."

A woman then approaches the kids gesturing to a seat across from them. "Do you mind if I sit?"

The demigods don't respond, but she still sits down placing an animal carrier down next to her. 

"You poor dears." She says, "Your parents aren't here, are they?"

The animal in the carrier whines and she turns her attention to it, "Isn't that right, precious? Don't children get scared when they're all alone?"

The four kids shift in their seats looking at each other anxiously. 

"It's okay. I'm a mom. I know how scared you must be." She turns to the guard outside their seats, "Excuse me. Would you mind giving us a little space? I think... I think you're making them nervous." 

"I want you to know, I don't actually think that you made that mess back there. I just wanted a moment alone with you. There are some things I need you to understand–" 

Eurydice cuts her off, "You have something on your jacket." 

"It looks like... It looks like glass." Annabeth points out. 

"No one smashed out the windows from inside our cabin." Grover figures it out, "Someone smashed them in from the outside." 

The woman hums before turning to the carrier again, "Oh! Yes, sweetheart. I know, I know. You're impatient."

The animal in the carrier starts growling. 

"But we're almost there," The woman continues before turning her attention back to the demigods, "This isn't your fault, but sadly, you're going to have to bear the burden of your parents' mistakes today." 

"Listen, lady." Percy challenges, "I don't know who you are, but I think I know what you are. We've run across a few monsters like you and we've sent them all packing."

"Monsters like me?" The woman asks, "Well... of course they're like me. They were my children."

"Children?" Percy asks, "What does that mean?"

"The Mother of Monsters." Eurydice says, "Echidna."

The monster in the carrier lets out a loud growl, but Echidna shushes it, "Monster. It's an odd word, considering my grandmother is your great grandmother, and this has always been a family story. But... to my eye, the demigod is the more dangerous creature." 

"Disruptive. Violent. If I exist for anything, it is to stand in the way of monsters like you."

She places a hand on the carrier, "My little one here. She's just a pup now. Bless her heart. Today... you will be her prey."

"Are you afraid yet? Oh. It's all right. Fear is natural. It's also essential to the hunt. Your fear. Your doubt. Your confusion." She lets out a low chuckle. "I needed you to understand what was happening so that she could track the scent."

"So that she could learn and grow, because... that's what a good mother does for her children." She taunts, "Not that you would know." 

Echidna opens the zipper on the carrier and the demigods jump onto their feet.

"You should run now," She tells the kids and they listen taking off. 

Before they get very far something hits Percy in the arm and he lets out a hiss. Eurydice grabs him to keep him running faster. The guards try to stop them, but they push past them and keep running. They make it to another train car and Annabeth puts a chain around the door to try and keep the others out. 

"Percy," Eurydice turns his attention on the injured boy. 

Grover notices something sticking out of his arm, "What is that?"

"It's a stinger," Annabeth answers, "Grover, do you know what kind of monster has one of those?"

"I don't know. I mean, nothing good probably." Grover answers.

"Do you feel okay?" Eurydice asks the boy concerned for him.

"I think so. Why?" Percy answers, "Do you think it's poisonous or something?"

"I'm not sure." Eurydice answers honestly. 

They look out the window and see Echidna and the monster moving closer and closer to them. 

"We gotta move," Eurydice stresses. The train PA announces that they are making an unexpected stop. 

The monster freezes no longer moving towards the group.

"Why isn't it still chasing us?" Percy asks.

"Echidna said whatever she was hiding in that carrier, it's young." Annabeth answers, "It won't venture too far from her mother. She's learning to hunt... and this seems like the hunting part."

"We aren't gonna be able to outrun them for very long." Grover points out.

"We don't need to. We just need a safe place to hole up." Annabeth tells them.

"Some place safe." Percy says, "Any ideas where we might find one of those?"

"I do." Annabeth answers, "A sanctuary, dedicated to Athena, built by one of her demigod children a long time ago."

"There is an Athenian temple hidden somewhere in the middle of downtown St. Louis?" Percy asks.

"Yes. Except it's not all that hidden."

The group approach the gateway arch and Eurydice laughs in disbelief. 

"It's 630 feet wide, 630 feet tall, both to within an inch. It's got no internal support. Each side is balanced perfectly against the other. The arch is held up by symmetry. It's held up by math." Annabeth tells the group. "And it's earthquake proof, so Poseidon can't ruin it."

"This is how you show Athena your love. A monument to the power of perfection." She tells Percy.

"It's a monument to some other stuff, too." Percy says gesturing to the tourists surrounding the arch. 

"You're talking about what some humans want this place to be about." Annabeth says, "I'm talking about what it actually is."

"Whatever." Percy concedes, "We're safe here, right?"

"No monsters can enter. Not even Echidna." Annabeth informs, "We're safe."

"Great." Eurydice responds before moving on with the plan now that they're safely in the arch, "Well, since our train exploded, I'm gonna see if there's another one we can get tickets on. We can't stay here forever." 

"Just because we're prey, doesn't mean we need to be helpless." Grover snaps walking off with Eurydice. 

"You okay?" Eurydice asks Grover. She has always been able to tell when the satyr is upset. 

"I will be," Grover says softly, "I'm just so tired of those two always fighting and ignoring us." 

The two get new train tickets before moving back towards Annabeth and Percy. 

"Percy!" Annabeth calls out as the young boy falls into her arms. The two run over as quickly as they can.

"Hey! What happened?" Grover asks.

"I think... I think those stinger things were poisonous." Annabeth explains, "I have an idea. Come on, help me." 

"Okay." Grover agrees and Eurydice grabs Percy following Annabeth outside to the fountain. 

Eurydice places him in the water and Annabeth begins to splash him, "The water cured him back at camp, it should work for poison too, right?"

Percy begins to stand up. 

"You know, I think it's I think it's working. This was a great call." Percy then falls back into the water splashing the other three half bloods. "Or not."

"Maybe it needs to be naturally running water for Poseidon to be able to heal him." Annabeth reasons. 

"Percy." Eurydice helps the boy up, concerned by how pale the boy is. 

The demigods attention is drawn to the city by the sound of car horns honking. They see echidna and the monster moving closer and closer.

"Okay. We need to get back inside." Eurydice says grabbing Percy and supporting his weight.

"No, we need to keep trying." Grover insists wanting to heal Percy.

"This isn't working." Eurydice tells the other sadly, "And she's coming."

"Okay, look, we'll take Percy inside, and we'll go the temple's altar." Annabeth tells the others and Eurydice nods.

"Altar? Where is there an altar?" Grover asks.

"The highest point, the best view." Annabeth says.

"Okay, but what good is that even gonna do us?" Grover asks nervously.

"We're gonna get to the altar and we're going to ask my mom for help." Annabeth tells them. 

"I thought we don't ask for help." Percy jokes weakly.

"Come on, let's get moving." Annabeth says before freezing in place, "Guys, d-did you hear that?"

"Hear what?" Eurydice asks concerned.

"Never mind. Come on." Annabeth shrugs it off. 

"What was that back there? What did you hear?" Percy asks Annabeth, "She spoke to you. Alecto did that with me back in the museum in New York. What did she say?"

The monster growls from outside the door. 

"Was that the Chimera? I think... I think that was the Chimera." Grover panics, "How did the Chimera even get inside here? How did any monster get inside here?"

"Annabeth?" Eurydice says softly, "We're in a sanctuary. Athena would have had to let her in, but why would she do that?"

"Annabeth!" Percy says, "What did Echidna say to you?"

"She said my impertinence wounded my mother's pride." Annabeth explains, "And that that will be my doom."

"Impertinence? What kind of" Grover starts before cutting himself off, "Medusa's head."

"I embarrassed my mother."

"But I'm the one who sent the head to Olympus." Percy defends, "I signed the note."

"And I went along with it." Annabeth argues, "It embarrassed her. Now, she's angry."

"Guys, what are we gonna do?" Grover asks.

"She isn't gonna help us when we get to the top to save Percy." Annabeth says sadly.

"No," Grover argues, "I meant, what are we gonna do about Echidna and Chimera? They're gonna be right behind us."

"We're not gonna have much time." Annabeth reasons, "They'll be up here any minute. And if my mother isn't going to protect us, then we'll just have to fight it out up here." 

"We gotta get everybody outta here." Eurydice tells the others. She walks over to a fire alarm on the wall and pulls it hoping that means they'll all evacuate. 

"You two and Percy follow them down." Annabeth tells everyone.

"What? No, no, no. W We're not splitting up." Grover argues.

"Grover, come on." Annabeth sighs.

"No." The satyr answers firmly, "No, no, no, no, no, we're all getting out of here together."

"We won't make it. The Chimera is the demigod killer." Annabeth argues, "Someone has to stay back to slow her down and buy everyone some time." 

"Come on." Grover protests.

"Help him down the stairs and get him to the river." Annabeth tells Eurydice and Grover, "And don't stop. Not till you get to Hades, not till you have the bolt, do you hear me?" 

"Okay, go." Annabeth tells them.

"Wait." Percy says pulling out his sword, "Take this." He hands the sword to Annabeth stepping out from Eurydice's grip. As she takes the sword he grabs her and pushes her out of the room shutting and locking the door. 

"Percy," Eurydice pleads, "Don't do this."

The three of them begin to bang on the door trying to get to Percy.

"Percy, no! Don't do this. They'll kill you." Annabeth begs.

 "Poseidon's never helped me before." Percy confides, "He wasn't gonna start now."

With a grunt of pain Percy moves further from the door, "I would've never made it to Hades."

The three kids on the other side of the door plead with him. 

"But you can." 

The chimera breaks through the door on the other side. 

"And now you will."

The three watch helplessly from behind the door as they watch the chimera and Echidna approach Percy. 

Percy disappears from their sight and Eurydice's heart sinks. She just lost her little brother, who she swore she would protect no matter what.