The tsunami isn't done wreaking havoc on the Santa Monica Pier on 9-1-1. Following the devastation of Monday night's episode, the waters have begun to change direction, and everyone who we thought might be safe will find themselves in jeopardy again — especially Buck (Oliver Stark) and little Christopher (Gavin McHugh).

The first episode of this two-part event saw Buck successfully rescue Eddie's (Ryan Guzman) son from the water and lead him to the safety of a floating fire truck, which also served as a launch point for him saving others who were stranded in the water. The two then enjoyed a brief moment of calm, after Buck gave aid to everyone he could, but then the second leg of this perilous journey began. The waters suddenly began their rapid recession back to the ocean, and as Buck attempted to help others who were thrown overboard, he was shocked to discover Christopher has also washed away from their makeshift raft and is now missing in the rushing water.

The third episode of 9-1-1 Season 3 will pick up with this devastating development, and while we don't yet know whether Buck will once again be able to find and save the tyke, TV Guide spoke to Oliver Stark about some of the biggest moments of the tsunami's initial impact, and he promised that the second part of the journey is bound to be even more emotionally gutting.

Gavin McHugh and Oliver Stark, <em>9-1-1</em>Gavin McHugh and Oliver Stark, 9-1-1

Even though Buck has health problems and he's been removed from the firefighting squad, he gets the chance to rescue people all the same as though he's on the job. Does that give him solace, something he's been missing?

Oliver Stark: 100 percent. He's a firefighter. That is who he is, and whether he's in a uniform with a badge or not, he will still step up to help other people. I don't even think there's a moment of deliberation. When he sees someone that needs help, and he's in that water, I think he knows his heart.

There's a really tender moment when the bodies start to float by and Buck grabs Charlie to shield his eyes. There's a lot of subtle emotion in that scene. Can you talk about that part?

Stark: It's so easy to create those moments with Gavin because he's so in it, and he's so real. He also understands what we're making. It kind of reminded me of that movie Life Is Beautiful where they're in the concentration camp and this dad plays with his son to get through it and to make him not realize what he's going through. ... He's in it right now, smiling and having fun, and he can be this ray of light for everyone out there on the fire truck. I really loved to shoot the scene with him, and it was actually really interesting how very much changed when we got there. I had never read it as though Buck was really asking for advice from Christopher, and as soon as I sat down with Gavin, it was like, everything changed in a great way. He just draws that type of emotion out. In the scene, Buck is really asking an 8-year-old for advice, and then he has to step back into the parent role and shield him.

Buck is up against a wall here because every time he goes to rescue someone there's more and more. And like you said, he doesn't hesitate to go for it. Do you think he's going to feel guilt and anguish when he goes to help others and loses Christopher in the process?

Stark: Absolutely. I had a lot of conversations with the director of the episode, Brad Buecker, about how it's almost worse when it's somebody else's child. Because if it's your own child and you lose them, you've destroyed their life and your own life. If it's somebody else's child, you've destroyed the child's life, your life, and your best friend's, in this case. So, I think that guilt is almost doubled that this is somebody else's child. I remember as well feeling like, 'Should I jump in the water after him? I don't know if that feels too much.' I don't have children, but Brad does, and he said, 'Listen, if my child went into that water, the next second, I don't care if it's suicide, I would be in the water after him.' And it was like, OK, in the water I go. So, I think there's a lot of guilt and a lot of treating it as if it were his own child.

What's ahead for the next episode with Buck and Christopher?

Stark: The second episode has a lot of action and high-octane moments, and the third episode still has some of those, and there are still big set pieces. But it's also an emotional journey. I've heard it's like the second act of The Impossible movie. The tsunami has hit. Now, it's the aftermath. Now there is all kinds of devastation left. Now they're trying to reconnect. It's gonna rip people's hearts out a little bit.

9-1-1 airs Mondays at 8/7c on Fox.