The episode opens in Dimension X where a creature that is frantically running inside a Kraang facility. This creature turns out to be Leatherhead. He spaces the building and attempts to hide so he can contact the Turtles with a Kraang Communicator. Through a brief transmission, he is able to tell the Turtles that the Kraang have perfected their mutagen. However, when Leatherhead is about to continue speaking, some Kraang soldiers manage to catch up to him and distract him. He successfully destroys the first round of Kraang, but reinforcements catch up to him and defeat him with stun guns.
Meanwhile, back in the Sewer Lair, Leo, Raph and Mikey are sitting down and watching the final episode of Super Robo Mecha Force Five, while Donnie is in his lab. However, Mikey wants to show Donnie what is happening in the episode and he accidentally pulls on the cassette player and ruins the tape. In the lab, Mikey walks up to Donnie and asks him to fix the tape. But Donnie is busy at the moment. Just then, Mikey is distracted by an enormous robotic hand in Donnie's lab and Donnie tells him that part of the incomplete Turtle Mech that he’s working on. But because the mech is so huge, he can't find a good power source tom power it. Donnie is so angry that he swipes a Kraang portal generator off of his desk, causing it to float up and become a gateway into Dimension X. Some air from Dimension X then starts to come into the lair. Donnie quickly gives filtration units to his brothers so that they can breathe it without passing out.
Suddenly, Donnie's Kraang Communicator activates and Leatherhead's transmission comes through, as his signal got to their Communicator when the Kraang portal opened up. The Turtles hear Leatherhead mention that the Mutagen is perfected and they listen up to the point where Leatherhead was captured. Mikey wants to hear more info and foolishly bangs the orb on a hard surface. The others are upset with him and Leo tells Mikey that, because of his screw-ups, he won't be coming to Dimension X with them. Mikey doesn't want to leave Leatherhead to be tortured and wants to fight alongside him again. Mikey then jumps into the air and goes through the portal, forcing Leo, Raph and Donnie to go after him right before the portal is closed.
When Leo, Raph and Donnie arrive in Dimension X, they find things to be very macabre and totally strange! Donnie immediately concludes that Dimension X has different laws of physics, as objects that are thrown return to you from the exact same angle...and you can also stand upside down. They then decide to start looking for Mikey, but they don't encounter him first thing. Instead, they come across two wiggly creatures made up of orbs that look small at first, but are actually big and menacing. The Turtles face off with these beasts, but are easily outmatched and electrocuted. However, the two creatures go away after the Turtles become silent. Donnie then notices a crystal that is similar to the Kraang’s power cell. But when he broke a piece of it off, the crystal glows and then makes a big explosion sending the turtles flying and crashing down with broken crystals. Leo then makes a new rule: "Don’t touch anything!"
The Turtles then arrive at a place where Mikey could be at, and Leo notices a small creature that looks quite cute and stinks up a storm. Raph warns Leo about Don’t touch anything, but Leo thinks that his little being is harmless. It's then revealed that this thing is a part of a massive insect-like creature. The alien attacks Donnie, Raph and Leo until Mikey, wearing a weird outfit, arrives and defeats it using nothing but noises, much to his brothers' shock. Mikey claims that he's been in Dimension X for months. At first, the other turtles don't understand him, but Donnie tells them that the time flow of Dimension X must be faster than that on Earth, causing a temporal differential. The alien creature then comes back, but Mikey scares it off again with a loud shriek. Donnie questions this ability and Mikey replies that things in the dimension are sensitive to sound. Mikey then collects some weird alien worms that, when squeezed, tongues emerge from them and stick to other surfaces like grappling hooks. Raph actually commends Mikey on his intelligence and tells him that he's a genius in Dimension X.
Mikey then leads his brothers to a gigantic Kraang compound. He tells them that there are thousands of Kraang in there and that is the least of their worries at the moment, as Traag and his partner, Granitor, who can expel ice from his mouth instead of lava, come and attack them. Mikey comes up with a plan, but his brothers don't pay any attention to him and attack the monsters. The giants are way too strong for them and nearly kill Leo, but Mikey rescues his brother and defeats the rock creatures with another shriek!! Leo and the others are still confused about Mikey's abilities and smart planning. Leo even gets a little jealous of Mikey when he says 'Lets move!', which is something that Leo always says...The brothers then stealthily invade the Kraang lair.
Once inside, they walk up to a room where they see an ordinary tree being turned into crystal (which makes up a big portion of Dimension X), with Kraang Prime observing through a screen. Donnie figures that the Kraang are planning to turn everything on Earth into it's equivalent in Dimension X, thereby giving the Kraang more freedom to function. Then, the Turtles find Leatherhead in a room where the Kraang are holding him hostage. They quickly destroy the Kraang and free Leatherhead, who now has a more aged appearance. Leatherhead tells them that he's been here for decades, spying on the Kraang. Leatherhead then leads them to another area of the compound where the Kraang's robotic bodies are manufactured. They all see hundreds of Kraang emerging from hallways and preparing to enter some portals going to New York City. Donnie wonders if they can do something about this, and Leatherhead tells him that they can go down fighting, but Donnie then thinks that their best course is a retreat. However,
Mikey orders Leo and Raph to block off the hallways, Donnie to shut down the portals, and Leatherhead to smash the Kraangs, a lot. As the gang leap into action, Leatherhead destroyed most of the Kraang solders, Leo and Raph manages to close all of the gates, before any Kraang-droids. Donnie has problems trying to close the portals, until Mikey step in and try it himself and program to leave one portal open to Earth. He told the team that it hold open for 30 seconds. Leatherhead wants to stay and fight the Kraang, However, Mikey doesn't want Leatherhead to be stuck in Dimension X again and kicks him into the portal, and his brothers soon follow him. Mikey was the last one to enter the portal, but right before it was close completely, he manages to grabs a big chuck of the crystal which cause the broken crystals to glow and make a super big explosion which cause the compound to collapse.
Back on Earth, Mikey gives the crystal to Donnie which makes s a good power source for Turtle Mech. Donnie thanks him for this and, while Mikey is upset about the loss of his intelligence back on Earth, Leo reveals that it was here that Mikey thought of grabbing the crystal. Raph then wonders how long it will be until the Kraang are ready to invade. With the time differential between Dimension X and Earth, Donnie says that it could happen in as little as a day. Leo states that a big storm is coming. With this, the Turtles and Leatherhead huddle up and get ready to fight for the big invasion.
"Donatello: Time passes faster here, there's a temporal differential.
Michelangelo: I love tempura."
"Leatherhead: Turtles, the kraang have perfected the mutagen, they're about to..."
"Leonardo - Mop Mop."
"Raphael - Mikey, you're like a genius here.
Michelangelo - Hey, in crazy-backwards land, crazy-backwards dude is King."
"Michelangelo - So I'll be a genius there too?
Donatello - Yes, Mikey. Thank you for finding the one mildly positive thing among ten billion screaming nightmares!"
"Leatherhead - I would rather die on my feet than live on my knees."
Donatello - Well I wouldn’t, I have knee pads!"
"Leatherhead - I think the phrase you’re looking for is pain in the..."
"Donatello - Mikey, you are a genius!
Michelangelo - Not anymore! Now I'm just back to being regular mess-everything-up Mikey...
Leonardo - No way! Saving that power cell was brilliant. You did that here.
Michelangelo - Yeah, I did! Maybe I'm not such a screw-up after all!"
"Donatello - That overload should have taken out their whole facility! Their invasion will be set back for months!
Raphael - Yeah, but how long is that with the time-differential thing?
Donatello - I don't know...hours? Maybe a day?"
"Leonardo - Guys, there's a storm coming. Are we ready to fight?"
Before Mikey leapt into the Dimension X portal, he longingly recalled a series of sepia-toned memories of his shared moments with Leatherhead in the past. The first two scenes were stills from the season one episode It Came From The Depths, and the third scene was a still from the episode TCRI, but the final scene was a previously unshown moment of Mikey and Leatherhead blissfully spooning asleep together on the Living Room floor.
Though once common in everyday photography, sepia-toning hasn't been a technological necessity in black-and-white photography for generations. In modern photography and film, the technique is mainly used in an artistic fashion to reflect certain deep moods.
A Theremin, an instrument popularly used in old B-rated science fiction films and shows in the 50's was playing throughout the episode.
The unusual laws of physics of Dimension X, with its tiny gravity wells and travel by leaps of faith from surface to surface, are extremely similar to the equally unusual in-game physics of the video game Super Mario Galaxy. and its sequel, Super Mario Galaxy 2.
When Donnie takes a piece of the Power Cell crystal the entire crystal structure explodes, including the piece Donnie took from it, but at the end when Mike takes a piece through the portal only the bigger structure explodes, the smaller piece remains intact.