April becomes a full Kunoichi while she and the turtles must deal with a new enemy along with some old enemies.
- Adapted Out: In regards to the 2003 City at War storyline, Hun is absent here when he was a major character in that arc. Promos for upcoming episodes have revealed Hun's presence, however, but for now, he sits out the actual "City at War" episode.
- Anti-Villain —> Anti-Hero: Karai, who no longer has the brain worm in her and has mentally (but not physically) recovered from her mutation in "Vengeance Is Mine" (which aired two years prior), has moved into Shredder's digs and with Shinigami is now trying to erase what's left of the Foot from New York.
- While allied with the Turtles, she still does not move in with them and remains a bit distant. While she mentions wanting to make her own path, her words toward Splinter heavily imply that it's because the guilt of what the Foot Clan has done along with her own actions (even when brainwashed) make her feel that she does not deserve to be with them until she redeems herself.
- In some ways Harsher in Hindsight when the viewer remembers that Karai would have been aware of everything that she was doing during the mind control, but unable to resist it.
- While allied with the Turtles, she still does not move in with them and remains a bit distant. While she mentions wanting to make her own path, her words toward Splinter heavily imply that it's because the guilt of what the Foot Clan has done along with her own actions (even when brainwashed) make her feel that she does not deserve to be with them until she redeems herself.
- Authority Equals Asskicking: In the first confrontation with Shinigami, Leonardo is the only one of the group who manages to land a hit.
- Breaking the Fourth Wall: At one point, Raph complains about the events of the episode occurring on a Sunday. The show airs new episodes on Sunday mornings.
- The Bus Came Back: This is the first episode in over a year to have Karai and Baxter Stockman; Karai was last seen in "The Fourfold Trap". Baxter, however, has a non-speaking role and doesn't appear until the very end of the episode.
- Continuity Nod:
- Karai has recovered mentally from her mutation back in "Vengeance Is Mine", and Splinter having ejected her brain worm back in "The Fourfold Trap."
- Timothy can be seen in Donnie's lab.
- Splinter began training April as a kuniochi way back in the first season.
- Dark Lord on Life Support: Splinter's attacks on Shredder have left him critically injured to the extent he is confinded to bed with a breathing mask, IV chain and several bandages and casts. When he hears of Karai and her full Heel–Face Turn, Shredder declares war and Stockman begins pumping Mutagen into him.
- Dumbass Has a Point: Mikey is the one to tell Karai that there's more to life than pursuing revenge.
- Evil Power Vacuum: When Leo observes the city is quiet tonight, Raph suspects it will get a whole lot louder when various gangsters and supervillains seek to take advantage of Shredder being out of the picture.
- Evil Sorceress: Shinigami, the newest member of the Foot Clan, is indicated as being one. It's a Red Herring, though. She works for Karai, and her initial fight with the turtles was just a test of her skills.
- Evolving Credits: The theme song removes Leo's speech about the Triceratons turning the lyrics back to normal. The visuals have again received a slight change. The villains segment now moves horizontally to the right with Shredder releasing his blades when he appears and shows some allies (the left side of this screen has Karai/Miwa and allies to the Turtles including Mona Lisa and Sal Commander, while the right has their enemies including the remaining Foot Clan members and the Newtralizer), Mikey attacks some Japanese ninjas while Leo slices two Chromedomes, April's tire swing scene shows her new uniform, and Karai and Shinigami are shown on a water tower.
- Groin Attack: When April perfects the Dragon's Tail attack, she uses it to give Tiger Claw a good old-fashioned kick in the nuts. It immediately immobilizes him.
- The Heavy: With Shredder out of commission, Tigerclaw is acting leader of Shredder's Foot Clan.
- In Name Only: The episode is titled "City at War", but much like its' 2003 counterpart, has little to do with the original storyline (or even the 2003 storyline) - instead, it serves as the beginning of the arc.
- Initiation Ceremony: Karai insists the first fight was just a way to test Shinigami's skills before bringing her into the fold.
- Irony: While fighting Tiger Claw, Fishface, and Rahzar, Donnie and Mikey, the B-Team, are still standing as well as hold their own against Fishface and Rahzar. Whereas Leo and Raph, the A-Team, are both overpowered and defeated by Tiger Claw. Raph is taken out in one hit.
- April defeats Tiger Claw while Karai, who mocked her earlier for being weak, was defeated by him twice. Shinigami, who beat April twice, was also taken out by him.
- It Amused Me: During their rematch, Shinigami initially just toys with April and trolls her for the fun of it. When April starts tapping into her own powers, though, Shinigami takes her out quickly rather than chance anything.
- Jerkass Has a Point: Raphael stops Donatello from intervening in the rematch, saying April has to fight her own battles. Given her self-confidence issues at the time, she no doubt would not appreciate any help during this particular moment.
- Kick the Dog: After Shinigami brings back April's stolen tanto, Karai proceeds to mock April for being weak.
- Kindhearted Cat Lover: Mikey squees over seeing a supposed cat.
- Meaningful Name: Shinigami is Japanese for "God of Death". It's highly implied she will be a major enemy of the Turtles, but ultimately turns out to not be one.
- Mood Whiplash: Splinter dramatically suggests the possibility that Shinigami is working for Shredder. He then nibbles on some cheese on a stick.
- Mythology Gag:
- April's new outfit bears resemblance to her 2007 action outfit, but moreso resembles the 2003 show's take on it in the Back to the Sewers season.
- Karai donning Shredder-like armor hearkens back to previous instances in the lore in which Karai replaced Oroku Saki as the Shredder.
- At the end of the episode, the Shredder begins having mutagen pumped into him to aid in his recovery. A notable example of a mutated Shredder in the past was the Super Shredder at the end of Secret of the Ooze.
- The title of the episode is a reference to a major arc in the comics and 2003 cartoon. Though this episode has little in the way of an actual gang war and the title is pretty unfitting, it does start an arc of a rivalry between two Foot Clans, and Raph mentions early on that many nefarious characters will likely try to take Shredder's territory.
- 2003 Karai debuted in that show's adaptation of "City at War". This show features Karai's proper reintroduction into the series after two years of being reduced to a living plot device and rarely appearing - and as an ally of the Turtles like 2003 Karai ended the arc as.
- Tigerclaw's conversation with Shredder at the end is reminiscent of the ending of the 2003 "City at War" arc, in which Karai spoke to the severely wounded Ch'Rell.
- No One Could Survive That!: Downplayed, but the Turtles don't see the Shredder or his followers returning any time soon if at all.
- Not-So-Harmless Villain: Karai from the perspective of Shredder's clan. She and Shinigami appear to be the only active members of her clan, which doesn't look like much compared to Shredder's horde of mutant lieutenants. By the end of the episode, however, Shinigami proves to be a vital asset to Karai and helps destroy a major weapons cache of the Foot, striking a blow to Shredder's struggling organization.
- Oh, Crap!!: As soon as the bombs start, Tiger Claw, Rahzar and Fishface try to escape. They get stuck in the doorway and Tiger Claw can only say "Oh no!" before it explodes, with all three having deadpanned stares.
- Pet the Dog: While Karai wasn't above mocking April for losing her tanto to Shinigami, she has Shinigami return the tanto when their fight escalates to the point where one of them could get seriously hurt.
- Rage Breaking Point: After getting kicked around and laughed at by Shinigami a second time around, April cracks and starts unleashing psychic blasts.
- Redeeming Replacement: Karai and her Foot Clan faction seek to be this to Shredder's own clan. She states outright that she seeks to rebuild the Foot under better circumstances and restore its honor after everything Shredder did to pervert it, all the while wearing his helmet and sitting in his throne while doing it.Karai: I want to rebuild the Foot. Bring back honor to a clan that Shredder poisoned for almost two decades.
- Shinigami: Karai's friend from Japan and first member of her faction of the Foot Clan, is named after these figures in Japanese Mythology. Donnie at one point is seen reading up the myth when trying to figure out more about her.
- Ship Tease: Mikey becomes infatuated with Shinigami, though she is not receptive.
- Shout-Out:
- Tiger Claw calls Karai a "clever girl."
- Xever refers to himself as "the most interesting fish in the world."
- Significant Wardrobe Shift: April now wears a black jumpsuit with yellow stripes to go with her full kunoichi status.
- Status Quo Is God: Played With. After two years of chaotic back and forth, Karai is still not back with Splinter, though now she recognized him as her father, seeks to kill Shredder, and rebuild the Foot as an honorable ally of the Hamato Clan. However, her motivation is partly still out of vengeance (which Splinter points out and she does not deny), but also because he wants Splinter to be proud of her.
- Strange Minds Think Alike: After leashing Rahzar, Shinigami makes a "walk the dog" one-liner. Mikey boasts he was about to the say the exact same thing and feels this is proof they have a lot in common.
- Tomboy and Girly Girl: Karai and Shinigami. Karai keeps her hair short and wears practical body armor, while her fighting style is simple and effective. Shini has long, flowing locks and dresses like a Hot Witch, while her fighting style is very fluid and almost dance-like.
- The Unapologetic: Subverted. Both Karai and Shinigami apologize (even bowing) to April for causing her trouble over her tanto. April forgives her and they appear to be on good terms now.
- "Well Done, Daughter!" Gal: Part of Karai's motivation over her leading the Foot Clan and going against the Shredder is in trying to make Splinter proud of her despite him saying he already is. It's implied she feels guilty over everything she has done and does not join the Hamato clan out of a sense of shame. The fact that the paternal love she got from Shredder her whole life was conditional certainly did not help.
- Wham Shot:
- Karai is seen in armor reminiscent of the Shredder when Shinigami reports back to her.
- Shredder is in confined to bed in critical condition and on life support in a cliffside house. After declaring war, Baxter Stockman uses a special IV to inject Mutagen into him.
- We get our first hint of April being corrupted by the fragment since "Riddle of the Ancient Aeons" foreshadowed it.
- What Happened to the Mouse?: In-Universe. The Foot fled New York City after the Shredder was critically wounded by Splinter during the Triceraton invasion.
- Casey is briefly seen during April's graduation ceremony in the cold open, yet he does not appear afterwards, when April goes on patrol with the Turtles.
- Witch with a Capital "B": How April addresses Shinigami before their first fight.
- Would Hit a Girl: Leo, much to Shinigami's surprise.
- You Are Better Than You Think You Are: After getting whupped by Shinigami the first time, April's self-confidence takes a massive hit. Leo tells her he knows from experience how self-doubt can cloud the mind and that she should believe in herself. Splinter also notes that losing fights happens and that the only real failure is giving up entirely.
