This episode grabs you from the start. Raph and Casey are on the rooftop searching for Karai, while Donnie is in his lab inventing a new weapon to fight the Krang when they attack. Leo is on the defensive trying to get his brothers on board to escape the city before the Krang begin their invasion. Donnie disagrees with his decision and the two start to butt heads. Meanwhile, April is entertaining her human friend Irma, her high school friend who always keeps asking April about her mysterious friends she keeps hiding from her. April laughs it off, but then they are ambushed by the Foot Clan. Feeling trapped, April makes a decision to take Irma down the sewers and hide in the Turtles lair. When they arrive, Irma turns into a Krang spy droid and sends a signal to the them that the Turtle’s liar is discovered. Multiple portals start to emerge and the Krang begin to enter from Dimension X. All over the city portals begin to pop up and thousands of Krang soldiers start pouring out. In addition to the soldiers, mechanized ships and walking robots come out of the portal as well. The invasion begins.
Back in the lair, Splinter tells them to go while he fights with the spy droid, who happens to be named Krang Sub Prime. He is a separate entity from the rest. His body is more complex and his demeanor is more humanlike. The rest of the group escape into the tunnels only to be attacked by the Krang, Leo creates a diversion in order for the others to get away. Little by little the group is getting smaller. Raph and Casey are out in the city trying to reunite with the group, and the Krang are all over the city wreaking havoc. To make matters worse, Splinter and his band of evil mutants are close by witnessing all the chaos unfold. With the Krang destroying the city, he can come in and pick up the pieces. The scenes keep jump back and forth to Leo fighting for his life trying to get back to his team, to Splinter also fighting his way out of the sewer. Everybody as at its wits end trying to stay alive with the Krang attacking them, to Shredder sending his Foot clan to seek out the Turtles wherever they may hide. To matters worse, the leader of the Krang, Krang Prime emerges from Dimension X using the perfected mutagen to transform everything from the organic and inorganic to mutuated creatures and things. While trying to escape the Krang with the Turtles and April, her father gets dowsed with the mutagen again. This guy never gets a break, does he? April has to watch her father again turn into a mutant right before her eyes. On top of that, she feels responsible for all of this. 
If she had not revealed the Turtles lair to Irma, the Krang would never have begun the invasion. Honestly I do blame her, but the invasion is inevitable. In time, the Krang will find a way to come through the portal and start the war.
Leo’s fight with the Foot Clan is epic. The Foot Clan attacks him from every direction, and he matches their moves with great agility. Shredder watches him nearby, and waits until the right moment. That moment came when Leo pauses for a minute trying to catch his breath. Then he sends Fishface, Ra-Zar, and Tiger claw to finish him. They almost succeed until Splinter and Leatherhead emerges from the sewers. Splinter and Shredder fight. While Leatherhead fights with the Foot. Shredder and Splinter’s fight is one for the history books. .
The Turtles finally reunite at Aprils apartment only to be ambushed by the Foot and Leo’s body is thrown into the apartment barely breathing. They escape barely with their lives. They make their decision to use Donnie’s invention to go on the offensive and attack the Krang. The Turtle-Mech is revealed. It is a cool thing to watch this monstrosity push back the Krang. They come face to face with Krang Prime. Their fight seems unevenly matched because of the size difference, but the Donnie uses his inventive imagination to defeat the Krang Prime. The brain that occupies Krang Prime is still alive and goes after the Turtles, but is crushed by Casey’s van when it rammed the brain down. They escape and head off to their hideout. At their lair, they decide to leave and head out of the city. They start grabbing things that they think they need. April gets a mental image of Splinter and Shredder fighting in the sewers and Splinter is in trouble. The two ninja masters are fighting to the death when the gang arrives. Splinter seems to be winning, until Shredder sneaks in a deceptive move that incapacitates Splinter. He takes that opportunity and defeats Splinter. He gets thrown down a large sewer drain. The Turtles can only watch their master go down. Shredder then leaves.
The final scene shows Turtles leaving the city, and Splinter being washed down the drain. A silvery creature grabs Splinter and takes him to safety. It is revealed that this creature is Karai. She then leaves into the sewers.
This is a double dose of the Turtles. It is explosive and exciting. The action is nonstop and relentless. The city is left in ruins, Splinter is lost in the sewers, Leo is seriously injured, and Shredder and his Foot clan are running loose in the city. A dark and gritty feel is left at the end of the episode, and I like that, can’t wait until the next episode.
