Here is how the rescue mission begins in The Overlord's Revenge.

At last, Po, SpongeBob, Emmet, Wyldstyle, Batman, Gandalf, Laval, Lloyd and their friends begin the rescue mission for the princesses when they came to Canterlot.

SpongeBob SquarePants: I hope we'll succeed our mission soon.

Machia: Not to worry, SpongeBob.

SpongeBob SquarePants: Machia, what're you doing here?

Machia: Well, I've come to help you with a project.

Po: What kind of project?

Lloyd Garmadon: I think we're about to find out.

Tigress: What do you have in mind?

Machia: I'll show it to you.

Po: Good thing she's on our side.

SpongeBob SquarePants: It sure is, Po.

Machia: Before we came to the past, I stole these.

Machia brings out the blueprints and it shows the Super Iron Doom instructions.

Lloyd Garmadon: It's the blueprints to the Iron Doom!

SpongeBob SquarePants: Yup, is that all you got, Machia?

Machia: Only this is a Super Iron Doom. It's like the Iron Doom, but with wings on the back and a horn on the head.

Spongebob Squarepants: I don't know what the Overlord's up to, but we've gotta stop him and the Storm King!

Queen Novo: Well then, what're we waiting for?

So, SpongeBob gathered his friends as he explain the plan.

Along the way, they have to fight off the Storm and Stone Armies to get to the princesses.

SpongeBob SquarePants: (puts on his karate gear) Safety first.

Batman: Ready when you are, SpongeBob.

SpongeBob SquarePants: Time to save Equestria!

Then, they started battling the Stone and Storm Armies.

Po: SpongeBob, duck!

SpongeBob SquarePants: (ducked as Po kicked on Storm Guard) Thanks, Po! (took out one Stone Guard)

Sandy Cheeks: Time to take it up a notch.

Tigress: Then, what'er we waiting for?

Batman: It's kick some blocks!

Emmet Brickowski: Awesome phrase!

Lloyd Garmadon: Ninja go!

So, they kept on getting close to Canterlot and to rescue Twilight and the princesses.

As for the Overlord, he arrives in a mech shaped like the Iron Doom called the Super Iron Doom.

The Overlord: I see your friends have come to rescue you.

Twilight Sparkle: You know it, Overlord!

Princess Celestia: Once they're threw with you, you'd be send to Tartarus for good!

The Overlord then uses the Super Iron Doom mech to grab the weakened princesses by their tails.

The Overlord: Here are the terms of surrender, princesses. Bow, and we'll destroy the heroic alliance, but spare the citizens. Refuse and we still destroy the heroic alliance, but the citizens will NOT be spared.

Princess Luna: We will never bow to you, Overlord!

Princess Cadance: Neither will we bow to the Storm King!

The Overlord: Then say goodbye to your subjects.

Then, the Overlord starts to try and destroy them when Po beat him to the ground giving SpongeBob the chance.

Po: Now, SpongeBob! Restore their magic with your Chi!

SpongeBob SquarePants: On it!

At last, SpongeBob used his Chi Technique and restore their Alicorn Magic.