A seriously understated one where, at one point, Donnie and Mikey cooperate flawlessly to get rid of some Kraang-droids. It might not seem like anything special out of context, but the first episode goes out of its way to point out that, in this universe, the turtles haven't been trained to fight as a team. Prior to this, they'd all been getting in the others' way all over the place.
April successfully infiltrates a Kraang hideout and finds out their plans to dump mutagen in the city's water supply. She almost gets caught by a Kraang droid, but she takes it out by bashing its head with a metal pipe, and knocking it out the window! Looks like she's well on her way to graduating from Distressed Damsel to Action Girl.
Donnie gets one too, by defeating Metalhead and the Kraang controlling it with an insult, fancy maneuvering, and a broken 'stick'.
The best part is that he does it still using his trademark genius, proving that not all science is lasers and robots; sometimes, it's just levers and simple physics.
The Turtles taking down the Kraang with arrows with lit dynamite sticks attached to them.
The Turtles minus Donatello taking on and soundly defeating Bradford and Xever.
That's not to say that Donnie doesn't get a moment of his own— once he's deactivated the mutagen bomb he makes a wide loop, smacking Bradford around before teaming up with Raph against Xever.
Raph gets the most props here, as up until this point, the Turtles had to pair up to fight with Bradford and Xever, but in a situation where they were down one, he performs nearly as well on his own as Leo and Mikey do together.
The Shredder's fight with the Turtles. Notable moments include Donnie's bo staff breaking on his armor, throwing Raph into a nearby electric sign and cutting the chain on Mikey's weapon when he tries to capture him.
Leo and Raph manage to beat Snakeweed as a team, and with ice.
Raph shouting "Booyakasha!" as he delivers the final blow. Doubles as a Crowning Moment of Heartwarming when you remember that it's Mikey's Catch Phrase, and how worried Raph was when Snakeweed knocked him unconscious earlier in the episode.
A brief one, but when Raph is overwhelmed by the pressure of being the leader when Mikey is knocked out, Donnie takes control of the situation and manages to contain Snakeweed, by himself, with nothing more than fancy manuevering and his staff.
Sid tries to attack Mikey from behind only for Mikey to wrap his nunchucks around his wrist and toss him into a wall. The best part was that while Mikey was aware of what he was doing, he wasn't even looking.
Then Stockman and Tsoi come up behind him. Mikey gives them a Death Glare and next we see them, Tsoi has been beaten unconscious and Baxter is backing away in fear.
Donatello had a pretty good showing in this episode. He managed to make an awesome vehicle that impressed even Raph, he cooked up anti-venom in minutes while in a bumpy ride, and he took charge of the situation more than once.
Like Dogpound, Fishface's first proper outing against the turtles is an impressive piece of work. His new legs make him even more versatile in fighting, and when Raphael pins the legs to the wall, Fishface takes a bite out of him.
April infiltrates TCRI, and hits a squad of Kraang droids surrounding her with a yo-yo. It's not enough to subdue them, but at least she managed to escape.
The turtles successfully fight the Kraang while holding their breath to avoid inhaling the toxic atmosphere.
Leatherhead's Big Damn Heroes moment where he single-handedly knocks down Traag when it took three Turtles to do the same thing. Followed almost immediately by his Heroic Sacrifice, which doubles as a Tear Jerker.
Raph saves Leo from the metal ball of doom Fishface throws at him. It's also ironic since during Splinter's training session, Leo was the only one who knew "when his enemy would strike" and avoided Splinter's attack, but would've been killed or at least severely injured had Raph not intervened.
Leo commanding his brothers along with Dogpound and Fishface in taking down Stockman's giant robot.
Baxter was able to make the Maze of Doom and another Humongous Mecha without a T-Pod's aid. Compared to the poorly-functioning armor he was wearing in his first appearance, it's pretty damn impressive.
Michelangelo manages to not only remember the instructions Donatello gave to him despite his usual problems with Donatello's technobabble, he, through sheer force of will (and a bit of self-harm), fights the mind control to complete the cure for his Brainwashed and Crazy brothers.
Remember back in the pilot when Raphael beat Donatello without his sai by using Donnie's own staff against him? Well, in The Teaser, with April watching, it's Donnie who gets the upper hand by using his Bo more creatively.
If he hadn't been so focused on impressing April, he probably would have won.
Mikey saving the day by running a car into Newtralizer.
Newtralizer himself is simply insane with how badass he is. His Establishing Character Moment involves decimating a squad of Kraang droids and eating a Kraang.
Donnie shows his true competence going solo, by using his intelligence in battle to make up for his lacking skill, compared to his brothers. One example involves attracting a Kraang droid to his ringing T-phone, where the last thing it sees before it's unconscious is a picture of Don waving at them.
In a "King of the Mountain" challenge that Leonardo decided to do to break up the boredom of collecting Mutagen, his brothers all demonstrate their distinctive traits in getting to the "mountain" that Leonardo was guarding.
Karai makes an attempt on April's life with her naginata. Donnie not only blocks it, but disarms and owns her in five seconds! Karai! Shredder's "daughter" and lieuteant who usually gives Leo conniptions.
Just as Karai is about to finish April, Donatello blocks her at the last moment shouting "You're not gonna touch her!" so heroic~ *starry eyes*
Mikey using his shadow puppet skills to distract the Kraang so that the other turtles can ambush them.
Also Mikey destroying the Kraang's robot with its own weapon.
Casey is able to defeat a small army of foot bots with only a hockey stick and pucks!
Just to show how far she has gone since the beginning, April gets several:
She holds her own against the Foot Bots and destroys several of them.
She runs away from Casey so the majority of the robots would follow her and not attack him.
She uses knives thrown in the wall as a ladder.
She helps Leo in taking down Karai.
Special mention to when Karai is mocking her,stating that she has way more experience and she is surely going to win...and then the tessen April tossed comes back like a boomerang and hits her in the face.
April:"You talk too much!" (does a taunt by fanning herself)
On the antagonist's side: The entire episode is about how awesome and dangerous Spike is after Slash decides to try and get rid of Raphael's brothers and shows that despite having no actual battle experience and only learning ninjitsu by watching others practice, he really is just as strong as he thinks he is. Not to mention tricks like mimicking Leo's voice (and shadow), which isn't clear if it is a part of his mutation or not, that make him much more intimidating.
It's interesting to think back on the pilot when Leo beat Raph in a single stroke battle. Replayed here, it appears that Leo's been leveling up, as he's the one who takes triumph in the turtle free for all. (At first.)
The fight between Raph and Casey. It's very fluid and fast paced. Props for Casey as well for holding his own against Raph despite having no training.
The battle against the Foot Bots. Especially since it gives April and Splinter some much needed fighting scenes.
Even better: after more than a full season, we finally have all 7 of the franchise's main protagonists fighting side-by-side. And it. Is. AWESOME!
Raph and Casey teaming up to stop the Foot Bot from giving away the location of the lair.
All of Slash and Newtralizer's fights against the Kraang. Seeing them get pummeled so effortlessly is always pretty cool.
Casey showing up to save the turtles from the Kraang Walker, then hitting a bomb into the exhaust port and blowing it up.
Raph saving Slash from getting crushed by the Walker, and then the two of them fighting side-by-side against the Kraang, doubles as a CMOH.
Slash turning against Newtralizer by trapping him in the Walker.
A minor one for Leo: after Raph explains the situations at the start of the episode, it only takes seconds for Leo to form a good patrol plan, and assigns everyone with their best partner and setting. (Raph and Casey to the alleys, so Casey's lack of ninja training won't slow them down, Mikey and Donnie are a team, so Donnie can keep Mikey on-track, and himself in the sewers where he'll be close enough to home base) He's clearly growing into his role as the leader.
Leo winning the fight for the pizza dumpling by waiting back for his brothers to subdue each other and then taking it at the last second.
Casey for being able to evade Tiger Claw for so long.
Mikey instantly coming up with a good plan to locate Casey. Raph was stunned.
And then knowing exactly where to lead Tiger Claw.
Karai attacking Tiger Claw to save Leo
Tiger Claw deserves some credit here too. He survived being eaten by a giant worm, fought his way through the '87 turtles, and fought through multiple dimensions to get back. Then over the course of the episode he beats up Casey, then takes on and defeats all four turtles, and immediatly afterwards Karai as well. Dude was kicking everyone's asses.
The turtles and their plan to rescue Karai. At first everything seems to go smoothly as they take out everything and everyone at Shredder's headquarters with some exceptions coming up. Tiger Claw let's it be know they're not leaving alive but Donnie is well prepared for him thanks to the species he became. Then a chase breaks out between the turtles and the Foot and Tiger Claw and just when the turtles give them the slip it seems they're about to fail only for Donnie to pull one more trick. And everyone of the turtles had a hand in helping with the plan as they played to their strengths.
Leonardo and Karai manage to catch each other off guard in ways that show just how well they know each other. Leo correctly predicted she would go after the Shredder and heads her off when she thought she'd evaded him. Then, Karai uses a small manner of seduction to catch him off guard and knock him out with a handful of blinding powder.
The whole episode is basically one CMOA for Mikey. He figures out how Dimension X works, leads the others in foiling the Kraang's plan, which gives the second invasion a brief delay and finds a power source for the new turtle mech.
A villainous one for the Kraang. At the start of the episode, they actually manage to subdue Leatherhead, who usually tears through them like they're nothing.
The one Kraangdroid that actually gives Raph a decent hand-to-hand fight.
Leatherhead's line "I would rather die on my feet than live on my knees!"