The ships of the Fleet of Fog. Front row (left to right): Super Battleship Yamato, I-401 Iona, Heavy Cruiser Takao; Back row (left to right): Battlecruiser Haruna, Battlecruiser Kongou, Battlecruiser Kirishima.
"Dreifach ist der Schritt der Zeit:
Zögernd kommt die Zukunft hergezogen,
Pfeilschnell ist das Jetzt entflogen,
Ewig still steht die Vergangenheit."
— Motto of I-401, by Friedrich Schiller in Sprüche des Konfuziusnote Threefold is the time's pace: the future comes not in haste, the present is gone arrow fast, eternally still remains the past. From Friedrich Schiller's "Sayings of Confucius"
Arpeggio of Blue Steel (Aoki Hagane no Arpeggio) is a manga by Ark Performance, centering on humanity, having been pushed back from global warming raising the sea level, and their conflict with the Fleet of Fog, archaic-looking WWII-era ships who have been using Game Breaker weapons and armor to ensure that humans can't set keel or pontoon into the world's oceans without risking getting blown to smithereens. The world's navies banded together 17 years before the opening chapters of the manga and were utterly annihilated.Since then, Japan's fortunes have gone downhill in a huge way: Japan relied on importing and exporting for both raw materials and income for its economy; without either, the nation's industry has fallen quite far. In this rather bleak future is one shining ray of hope: Blue Steel, a bunch of ragtag misfits led by their captain, one Gunzou Chihaya. Having somehow acquired a Fog ship of their own, the Fog submarine I-401, only Chihaya and his crew can actually claim Fleet of Fog kills in battle.However, the I-401 and her crew have more enemies than just the Fleet of Fog: The Japanese government are very keen on getting their hands on what initially was their ship. Bureaucrats look at I-401 and drool. However, what nobody realized is that I-401, like many ships now in the Fleet of Fog, has a mind of her own, and a body to fit.An anime series sublined Ars Nova was aired during the Fall 2013 Anime season. The reaction to it has been mixed, to say the least.The anime is streaming on Crunchyroll. Viewers living in the following areas may watch it legally there: North America, UK, Ireland, Australia, New Zealand, Netherlands, Scandinavia, South Africa, Central America, South America, Caribbean, Turkey, Italy, Austria, Germany, Lichtenstein, Luxembourg, Switzerland, Belgium, Poland, Denmark, Czech Republic, Hungary, and Romania.Compare to Kantai Collection, another series (with an upcoming anime to boot) that also features female personifications of World War 2 ships. The two franchises have collaborated on an in-game event for December 2013, which features characters from the Ars Nova anime assisting the girls from Kancolle against Kongou and Maya.
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Tropes shared between media
A.I. Is a Crapshoot: Somewhat inverted: Before getting 'Mental Models', the Fleet of Fog ships were pretty straightforward about their goals and how they went about achieving them. Now that they have them, however, they're finding that alongside having the point of view of humans, they're also starting to pick up quirks like humans too...
Alternate History: The manga takes place in a setting where Russia masters supercavitation technology in the 1990s, and supercavitation torpedoes are invented. These torpedoes essentially make modern navies obsolete, and then the Fleet of Fog showed up.
In the real world, the VA-11 Shkval has already been in service since the late 1990s with the Soviet, and later Russian Navy. However, current supercavitation torpedoes like the Shkval have to slow down in order to maneuver or use their guidance systems, negating their primary speed advantage.
And this is exactly what the authors meant. In the manga's timeline, it took another 20+ years for supercavitating torpedoes to advance to the point where they dominated naval warfare and forced a complete revision of naval combat doctrine.
In fact, Bismark was revived as a Fog Battleship shortly before Germany's surrender. Though she does not help the Germans despite she is utterly invulnerable then.
Though interestingly, the Bismarck had always been intentionally classified as a 'he' by the Kriegsmarine. It nonetheless received twin blond girls as its mental models.
Aluminium Christmas Trees: Apart from rocket propelled torpedoes, the JMSDF attack submarine Hakugei has a rocket propulsion engine. DARPA has been researching adapting rockets to submarines for the last 10 years.
Anthropomorphic Personification: Fleet of Fog ships of heavy cruiser class and up (including the I-400 class submarines) all have one. Iona is shown having at least 9 moving around. They represent the 'Union Core', which may or may not be the AI that runs the ship in question.
The Union Core is confirmed to be the AI that runs the ship; when Kirishima is destroyed, Haruna manages to save her core. Prior to Haruna recreating a body for Kirishima via nanomaterial donation, the only way Kirishima could communicate at all was through the joint tactical network.
Applied Phlebotinum: The Fog Fleet is made of nanomaterial which can be formed into anything from hull plating to laser cannons as well as Mental Models.
Author Appeal: Want badass battleships and gorgeous women? The battleships are the gorgeous women!
The first chapter opens with one of these, having Iona atop the I-401 rising up from the water and crushing a concrete pier to save her captain and his charge, terrify the living daylights out of some gangsters, and cause some property damage to boot!
Iona does this for Haruna, Kirishima, and Makie in episode 6 of the anime, after Haruna pleads for someone, anyone, to save them from the soldiers attacking them. While Haruna could have easily killed them all, she took great pains to avoid killing them for Makie's sake.
Black Box: Most of the Fleet of Fog technology is described as this. Even though they had I-401 in their possession, neither the Japanese government nor its crew actually know how its specific components work. One example is the Corrosive Torpedo, which they have a very limited amount of and have no means of reproducing.
Breakout Character: Heavy Cruiser Takao is literally the most popular character. She's taken over the fandom so much that she's become the face of most advertisement, and she's on the anime's title splash for Crunchyroll. Even Ark Performance was surprised at her popularity, and in a magazine interview revealed that they had much different plans for her, but had to change their minds as a result.
Haruna is taking her own plotline within the manga.
Brits with Battleships: Recent chapters reveal that the Fog Fleet contains some Royal Navy model ships within its composition. Among them are the battleships Hood and Prince of Wales, the battlecruiser Repulse and the destroyer Vampire.
Iona has occasional shades of this. This effect is amplified with the mental models of 400 and 402. And completely averted with Takao's mini versions, who are moe incarnate.
Invoked with Musashi, especially in chapter 48.
Coat Cape: Haruna wears a heavy one covering her all up to nose. Official art parodies it by explaining the coat contains multiple pouches for mostly useless utensils.
Combining Mecha: Fleet of Fog warships are capable of doing this. Takao linked up with I-501 in order to improve her anti-submarine capability, while Kirishima and Haruma combined both of their Wave Motion Guns into a single massive Wave Motion Gun.
Emphasized more in Ars Nova with the titular vessel and also Kongou's Death Star..
Cool Mask: First Mate Sou has two; the first allows him to interface directly to 401's systems, likely equipped with a Heads-Up Display. He has a second, smaller one to use when maintaining the first. It is possibly because his face was damaged by the fire at the naval academy.
Crapsack World: The current situation is strangling Japan to death economically (see Easy Logistics), and Europe is said to be in the midst of a civil war.
Hyuga defects to Gunzou's force after being defeated, due to her fixation on Iona-sama.
Takao looks likely to become this as well, on top of her Stalker with a Crush attitude towards Gunzou.
Haruna and Kirishima help (in anime only) Iona and Gunzou out after the latter help save them from soldiers attempting to kill Makie.
Did You Just Punch Out Cthulhu?: The Hakugei III becomes the first human made ship to sink not only one, but TWO Fleet of Fog battleships. Granted, they used stolen Fleet of Fog tech, but it was still a very big deal.
Eaglelanders: The remnants of United States Forces Japan. Though they've long been isolated from their homeland, it's pretty clear that they retain their American traits; even their character designs (or at least Cruz's) are distinct from the Japanese cast members. Initially presented as Type 2 with their own agendas, they've since shifted into Type 1 (or at least positive Type 3) territory, being otherwise trustworthy and indirectly supportive of Blue Steel and Kamikage against Kita's faction of the Japanese government. One would expect this to be a result of leftover loyalty to America, but according to their commander, they're loyal to Japan and now consider it their home countrynote as many American soldiers ended up marrying Japanese nationals and starting families after they were left behind. In that regard, their motives are still questionable, but one has to admit this is a switch from usual depictions of Americans.
Easy Logistics: Subverted. The Fleet of Fog's interdiction of sea and air transport routes is adversely effecting Japan's economy, being an island nation reliant on trade (a repeat of what happened in WW2). Other nations are also stated to be affected; the one nation that seems to be doing alright despite the Fog is the United States, as it's one of the few nations that are completely self-sufficient.
Subverted again with Zordan's fleet. To keep his midget sub force constantly supplied (they can only carry two torpedoes each), he has to drag around an automated Type XIV Milchkuhnote Milk Cow U-boat for reloading and nanomaterial revitalization. This turns out to be the one major flaw in his strategy, as later on the I-401 snipes the Milchkuh with an enhanced Wave Motion Gun, thereby depriving the U-2501 of its main ammunition and fuel source and forcing it to prematurely withdraw.
Elaborate Underground Base: At least one underground drydock is seen operated by the Japanese Navy, located at Yokosuka; the characters note that it was built to protect what's left of the Japanese Navy from the Fog.
Enemy Civil War: The Fog Fleet isn't as unified an entity as it was a decade before, as the creation of Mental Models has given rise to individual personalities among its ships... and they don't always agree with each other. Depending on who you ask, there are as many as six different factions in the Fog Fleet alone, all with different aims and at times working at cross-purposes — like the Scarlet Fleet under Musashi and Shouzou Chihaya, and the faction under the HMS Hood, who aims to stop Chihaya's ambition rather than follow the Admiralty Code's orders.
Four Is Death: The fire had to hit Facility No. 4 and cause a lot of fatalities, of all places.
In-universe, the weapons, armor and shielding of the Fleet of Fog fall right into this trope. Humanity's combined fleets couldn't put a dent in them 17 years ago.
Before the Fog showed up, it was the introduction of Supercavitation technology for torpedoes. This made torpedoes capable of going more than 300 knots the standard of naval warfare. Ironically, in the real world, supercavitating torpedoes aren't yet gamebreakers, as their high speed is achieved at the expense of target tracking ability (the Shkval is an unguided torp) and range - 13km for the VA-11 Shkval, vs 38km for the ADCAP.
Gender-Blender Name: Takao, Kongou, and Musashi are not normally used as a girl's first name, although in a possible subversion those name refers to the ships, which just happen to use a female mental model.
And the Bismarck had always been explicitly referred to as a 'he' by its commander Capt. Ernst Lindemann and the Kriegsmarine. It still has twin girls as mental models.
Godzilla Threshold: Humanity has reached such a desperate point that they're willing to try anything in an attempt to fight off the Fog ships, such as utilizing one of the Fog's own ships against them, and later developing weapons designed to bypass or break through the Klein field barriers.
Humans Are Special mixed with Bastards: Yamato is keenly aware the only reason the Fleet of Fog is dominating the world's oceans is because of their Game BreakerWave Motion Gun, and the moment humanity catches up, which will eventually happen, human tactics and strategy with equal weapons will mean the Fleet of Fog is screwed.
Imagine Spot: In chapter 9 of the manga, Takao reads a children's book about sea captains. The result is... Interesting.
Due to the nature of the conflict with the Fleet of Fog, the Japanese Army has largely been ignored in favor of the Navy, much to their chagrin. This is the primary motivating factor why they've aligned with Kita, since they're desperate to win some recognition.
Even the Fog have this, as certain ships of the Oriental and Europeannote i.e. Pacific and Atlantic fleets hold subtle but distinct animosity toward each other. The best example of this is shown in recent chapters, where I-402, seemingly on orders from Yamato, actively supported both Takao and I-401 in fighting U-2501. And before that, there were assorted scenes of Oriental ships not being keen on U-2501's presence in the pacific, nor Souzou Chihaya holding command of the European Fleet in general.
Even within the Japanese government. When the Japanese government learns Haruna is spotted at Makie Osakabe's home, Central Prime Minister Mitsumine gives the Army the green light to assassinate Makie to prevent the Vibration Torpedo's plans being revealed to the Fleet of Fog. However, Makie is the Northern Prime Minister Makoto Osakabe's younger sister, and he sends cyborg commandos to help Haruna and rescue Makie.
The Japanese Army and Navy really don't get along very well; at one point, a firefight nearly breaks out between Navy SEALs and Army soldiers.
This is Truth in Television. During World War II, the Japanese Army and Navy often had to compete for resources and as a result absolutely despised each other. They would even go so sabotage each others' projects during wartime for no other reason but pure spite. And in the decades prior to World War II, both services were assassinating each others' officers.
Ironic Echo: Gunzou use one in his dialog with Ryokan Kita.
Kaiju Defense Force: The Japanese Maritime Self Defense Force and Ground Self Defense Force are upgraded into an official Navy and Army against the Fog.
Gunzou Chihaya was originally an officer cadet, and his father was a JMSDF Commander, who was the first captain of I-401 after she was captured by the Japanese Navy.
The JMSDF appears in several instances, being mostly helpful to I-401's crew - it helps that the CNO was a friend of Souzou Chihaya. The prototype submarine Hakugei assists in I-401's takedown of Kirishima and Haruna.
Laser Sight: Averted. The snipers are using infrared beam which are invisible to the human eye. This makes a lot more sense than optical lasers.
MacGuffin: The series actually has two: The vibration torpedo which must be smuggled to the United States to be mass produced, and the "Admiralty Code" that the Fleet of Fog is looking for.
Living MacGuffin: Given how the Admiralty Code has been depicted as having something similar to a Mental Model (a young woman wearing an old-style Apollo astronaut's rig), it could be seen as this as well.
Macross Missile Massacre: One of the tactics used by Takao, who puts up an impressive volume of fire for a mere heavy cruiser. She's later on the receiving end when approaching Iwo Jima.
In the anime, everyone uses this, since Fog ships can tank a lot of fire, even from Fog weapons.
Meido: The Mental Model of the battlecruiser Repulse.
Mugging the Monster: A Mental Model without her battleship. An entire army division sent to capture her. What could possibly go wrong?
Nazis with Gnarly Weapons: It seems the Fog Fleet's European forces are made up of Kriegsmarine vessels, ranging from the battleships Bismarck and Tirpitz to the Type XXI U-boat U-2501. The latter is commanded by Zordan Stark, a German subordinate (protegee?) of Gunzou's father.
No, Mr. Bond, I Expect You to Dine: Gunzou and his crew are treated this way by Ryokan Kita in episode 3. The latter, a former Japanese Naval officer, attempts to take away I-401 from him, but he refuses to do so.
Noodle Incident: Everybody talks about how the I-401 and its crew sunk the Hyuuga, but we've never actually seen the battle nor have we been given an accurate summary of events.
The fire at Facility No.4 of the naval academy Gunzo went to. The only thing sure is that Kotono is supposed to have died there.
Orcus on His Throne: The Fleet of Fog are shown to be perfectly capable of wiping out literally every coastal city on the planet (which basically means all of Japan). However, they're mostly content with just sitting back and maintaining the global blockade. Revealed by Yamato later on to be one of the orders given to the Fog Fleet by the Admiralty Code. Many among the Fog are now wondering why this is so.
Oh Crap: Lawrence, Makie's butler, has this reaction when he learns that the friend Makie brought home is Haruna's mental model.
Both Kamikage and the Prime Minister are willing to help the I-401 in any way they can, especially when a large portion of the Japanese government is out to try and thwart their plans.
Makoto Osakabe, the minister of Hokkaido, is perfectly willing to let Haruna, Kirishima, and Maya be, though he does admit that he will eventually have to put on a token amount of resistance to save face once they are ready to leave port.
The Reveal: Yamato's mental models are the splitting image of Kotono Amaha, one of Gunzou Chihaya's supposedly dead Childhood Friends and likely love interest.
On one side in the Japanese government is Assistant Secretary Ryuujirou Kamikage who wants to use the I-401 to transport their most successful man-made weapon that is capable of destroying a Fleet of Fog ship to the United States, so it can be mass produces, and has the support of the Japanese Navy and the local US forces. On the other side is the powerful Tycoon and top candidate for Prime Minister Ryoukan Kita, who wants the I-401 returned back to the Japanese Government so they can reverse engineer it and make their own Fleet of Fog ships, and who has strong contacts withing the Japanese Army.
At one point, an Army company under Kita's influence seizes the I-401's dock and arrests the crew; the Navy SEALs came very close to starting a firefight and rescuing the crew by force, before both sides were ordered to stand down.
Several run the Osakabe mansion and are realistic enough that they aren't revealed as such for several chapters.
Battlecruiser Repulse's mental model is an English maid. She takes the maid persona so far that she actually repairs her battle damage by sweeping, wielding Klein Field-projecting brooms with prehensile ribbons attached to her clothes.
Rotating Protagonist: The character focus switches so much that it's hard to seriously believe that Gunzou and the crew of Iona are the heroes here. Several Fog ships have shown more obvious character development.
Scenery Porn: Ark Performance shows what drawing Gundam manga for years does for skill at drawing mecha and landscapes. The 2400p versions of the scanlated manga are recommended to see just how much detail they put into everything.
Screw the Rules, I Have Money!: Invoked by Makie when a cab driver attempts to protest the apparent little girl trying to catch his cab by herself. His reaction to her SPECIAL GOLD CASH CARD is priceless.
Shout-Out: All of the Fleet of Fog ships are replicas of real life WWII era warships. The catch is of course, they're all based on ships that have been known to have sunk during World War 2...
Show Within a Show: Hisonic Miku-chan, an anime popular with Makie Osakabe, Kyouhei and Iona (AIs like anime, too).
Transforming Mecha: The Fog Fleet ships are all replicas of WWII warships only on the outside—under that is their advanced weaponry that they rearrange themselves to use.
Tron Lines: Fog ships tend to be covered in glowing patterns like tattoos.
Unobtainium: Thanatonium used for Fog Fleet corrosive torpedo/missile warheads.
Unskilled, but Strong: The fleet of Fog is a good example of this. Their Game Breaker technology is the source of their victory over humanity — if humans had it, they would have won through superior tactics. They are however very fast learners. As the story goes, Fog ships begin to work on developing tactics, though they still have a bias towards efficiency (this comes to bite Takao in the ass when she reveals her hand in her first engagement with Gunzou).
Takao suffers this after getting defeated by Gunzou, but compensates by picking up a huge crush for him.
Haruna has one after she learns the concept of death. She sucessfully saves Kirishima afterwards.
Villainous Rescue: Haruna and Kirishima's attack on Yokosuka provides a very convenient excuse for Gunzou to escape from Ryokan Kita's attempts to steal I-401 away from him. That he also successfully defeats both of them helps as well.
Wave Motion Gun: While the Fleet of Fog on the whole has dramatically superior armaments and shielding to the human fleets, Battleship-class vessels come equipped with 'Super Gravity Cannons', which despite the name fill the trope nicely, since the weapon tends to bend space around it and it's firing path.
We Can Rule Together: After cornering the 401 and its crew, Zordan suggests Chihaya to join the Fleet of Fog under his boss, Chihaya's father.
What Measure Is a Non-Human?: When Gunzou realizes that some of the Fleet of Fog ships are self aware like Iona, he decides to spare Takao rather than kill her.
Wolfpack Boss: Appropriately, Zordan's strategy. Alongside the U-2501, he also commands a force of automated midget subs known as the Seehund (Sea Hounds), which he uses as forward attack ships. Alongside, he can sacrifice several of these subs to create functional replicas of the U-2501, complete with torpedoes and other special features.
The US Pacific Forces who were stranded in Japan; as Fleet of Fog interdiction of air and sea routes makes returning them to the US impossible, they've been absorbed into the JMSDF as the Unified Forces.
In chapter 36, a young girl (who very much looks like a Mental Model) by the name of Archer appears before Britain signs over (surrenders) to Souzou Chihaya's Fog Fleet detachment. Though nothing has been elaborated yet, this girl could possibly be the Mental Model of the USS Archer-Fish (SS-311), the Balao-class submarine that famously sunk the Japanese carrier Shinano. Wouldn't be a stretch given the emphasis on submarines so far.
You Are Number Six: I-401 is the ship's initial designation, though her mental model is called Iona by the/her crew. Later on, we find she's one of a series of submarines, as we meet 400 and 402, who look exactly like her, though dressed differently. Also there's I-501, who was spotting for Takao.
Batman Gambit: When escaping from Iwo Jima, Kongou becomes obsessed with killing Gunzou, believing that he's the reason why several of the Fog ships formerly allied with her have done a Heel-Face Turn. While Iona attempts to escape through a carefully plotted route along an minefield, Kongou suffers massive damage pursuing Iona and Gunzou, while barely paying any attention to Takao's ship. After a successful hack by Hyuuga, she manages to divert Kongou's second attack from hitting Iona, and only then does Takao tell her former boss that the vibration torpedo was actually aboard her ship, and Iona was actually a decoy. Unfortunately however, Iona is attacked by I-400 and I-402 shortly after they escape from Kongou.
Body Horror: By Fleet of Fog standard, anyway. Kongou devours a huge number of Fog ships to become something that can't be called a ship, a serious transgression against Admiralty Code. Ironically, this is due to her obsession to destroy Iona which she sees as a stray.
Broad Strokes: The anime isn't really following the manga faithfully, to say the least.
Adaptational Villainy: Assistant Secretary Kamikage is far more ruthless in Ars Nova, personally ordering the assassination of Makie just to prevent the Fog from getting the secrets of the torpedo she created. Later it is revealed it was part of Kamikage's plan to have the I-401 crew to rescue and bring Makie with them.
Maya was never shown to have fought human-aligned forces in the manga (instead playing chaperone to both Haruna and Kirishima), however the anime has her join Kongou against her sister ship Takao and her allies in the Blue Fleet.
As the left and right hands of Yamato, I-400 and I-402 were never in opposition to Iona, and since they were loyal to Yamato, didn't even pay lip service to the Admiralty Code. In Ars Nova however, they seem to be unthinking drones loyal to the Admiralty Code, even more than Kongou... to the point that they attack Iona just as she escapes Kongou, detain Kongou due to her hate making her deviate from Admiralty Code, reveal Maya to be a fake Mental Model....
Adapted Out: The fate so far of Yamato, Kotono Amaha, Lawrence, the Hakugei, the mini Mental Models...
Composite Character: To a degree, this is what happened. The most apparent instances so far are Yamato's lines and role being given to Kongou, and Kamikage takes the role of the perpetrator of the attack on Osakabe household instead of Kita (though in the manga Kita wasn't the one who initiated it, he only takes the blame). Makie also gains her older brother Makoto's dependency on special medications to be able to digest the food she eats. Lastly, while this is dubious (as her full capabilities has yet to be disclosed), Iona in Ars Nova also gained the 'overdrive mode' employed by Hakugei in the manga.
Death by Adaptation: Maya, as it turned out, never was alive to begin with by Mental Model standards, and I-400 and I-402 are sunk in battle with Ars Nova I-401. To a lesser extent, this happened to Saori and Shouzou Chihaya as they died in the Facility No.4 fire.
Pragmatic Adaptation: A whole lot of plot points are excised outright from the anime adaptation, likely due to the 12-episode constraint the show has to deal with. This includes but is not limited to: the survival and Face-Heel Turn of Gunzou's father, Gunzou's mother Saori and her later interaction with Takao, the destruction of Institute 4, everything related to Yamato, Lawrence and the Osakabe household...
Spared by the Adaptation: In the manga, the Osakabe mansion maids are robots who were killed fighting against the Japanese Army. In the anime, the maids are implied to be possibly human or mere domestic robots, and they surrender peacefully to the Army.
In episode 4, as Haruna and Kirishima deliver a Curb-Stomp Battle to some human ships, the former overhears one man over the radio talking about not wanting to die and to be saved. She logs that definition in for future reference. In a bit of Irony, when I-401 delivers a Curb-Stomp Battle to both of them at the end of the episode, right before she's hit, Haruna also mentions not wanting to die much in the same way the unnamed sailor she heard earlier.
In episode 7, Hyuuga tells Takao that humans will sometimes perform a Heroic Sacrifice for the greater good, rather than think only about themselves. So when Takao searches for and finds Iona's core inside of Gunzou's escape pod in episode 10, she sacrifices her nanomaterials in order to allow Iona to reconstruct herself and allow Gunzou to survive.
Conservation of Ninjutsu: In episode 11, Gunzou is able to wipe out well over 20 Fog ships with one sweeping attack. Contrast that to the earlier battles where the humans on I-401 often had trouble against just one or two Fog ships.
Conspicuous CG: THE ENTIRE SERIES was animated in 3D CG, making the parts that are not CG conspicuous.
Cooldown Hug: Iona gives one to a crying Kongou in episode 12 after the former manages to breach the barriers put up by the latter.
Despair Event Horizon: Kongou is driven to one after The Reveal that she was seen as increasingly unstable, and especially that Maya was a spy for I-400 and I-402. After she manages to free herself, she combines with Maya's ship, and fires on her own allies in her zeal to kill Iona and Gunzou.
Earn Your Happy Ending: In episode 12, knowing they would lose in a face to face battle with Kongou, Iona instead suggests sending her core directly onto Kongou's ship, where she could attempt to connect directly. While risky, Gunzou doesn't have any other options, and Kongou doesn't make it easy for Iona either once the latter reaches the former's ship.
Evolving Credits: At the end of the opening sequence, scenes from that episode are briefly shown, such as the fight with Takao in episode 2. Additionally, Kirishima's teddy bear shows up when she's shown onscreen during the opening after she gains control of it.
Fashionable Asymmetry: Kirishima's outfit in the anime, taken from what she wore on one of the compilation volume covers. She never wore it in the actual manga, and had something far more conservative on instead.
The Glomp: Takao does this to Gunzou after Hyuuga restores her mental model in episode 12. Her impact knocks him to the ground.
Gecko Ending: The final episode of Ars Nova amounts to this. Not only was the Vibration Torpedo, its blueprints, and its creator delivered safely to the United States, everyone is presumed to live their lives peacefully from that point on, with even Takao's Mental Model restored. Even Kongou seems to be at peace with herself. None of this happens in the manga.
Ill Boy: Instead of masquerading as a butler, Toojurou Osakabe is this in the anime.
Irony: Kongou's obsession with terminating Iona and her zeal for upholding the Admiralty Code... is considered to be in violation of the Admiralty Code.
Killed Off for Real: The fate for some characters in the adaptation, specifically both Gunzou's parents.
Locked in a Freezer: The plot for Episode 10. Gunzou and Iona are stuck on the ocean floor in a broken ship, the life support system gets progressively worse, and there's even frost at one point.
Mid-Season Upgrade: I-401 merges with Takao in episode 10. Kongou merges with Maya, turning herself into a Death Star.
Mooks: The poor, poor, Nagara-class ships seemed to be given this treatment in Ars Nova, especially in episode 11, where 22 of them are obliterated in one pass by the Ars Nova.
Naval Blockade: What the Fog ships are doing to humanity. It's never made clear exactly what the motive for doing it is however, at least in the anime.
Nice Job Breaking It, Hero: Iona attempts to draw Kongo into conversation, talking about fun as the Mental Models hang out on the beach. This causes Kongo to solidify her resolve to destroy them all.
Kongo: "We are weapons. Weapons have no need of fun."
Also, her attempts to get Kongo to feel emotions and thus break away from the Admiralty Code backfire: Kongo remains determined to follow the Admiralty Code, despite the emotion she feels. What emotion is it? Hate.
Properly Paranoid: Kongo in episode 8. Invited to a parley by Gunzou, she keeps her guard up, does not allow herself to form an emotional connection, doesn't eat or drink anything that's served to her, and kept her Union Core on her foredeck and sent a decoy body to the parley, all to avoid being contaminated by Gunzou and Iona. Unfortunately for her, it didn't quite work.
Shout-Out: The music video for the opening song of Ars Nova has scenes shot in front of and aboard the JMSDF ship Kirishima.
Talking Is a Free Action: Several of the Fog ships find time to talk to and have tea with each other, even while actively fighting or evading each other at the time, such as when Iona tries talking to Kongou while escaping her at Iwo Jima. Justified in that it takes place in what amounts to a chat room and they are AI constructs.
There Is No Kill Like Overkill: Kongou sends 512 corrosive torpedos at I-401 in episode 12, while simultaneously using her Wave Motion Gun on them. When Iona attempts to reach her ship, she also sends multiple torpedos to stop her, and uses a Storm of Blades after the former manages to make it to the ship.
Title Drop: Iona reveals early in episode 11 that the Takao-Iona fusion is indeed called Ars Nova.
Too Dumb to Live: 400 and 402 shows shades of Artificial Stupidity towards the series' end. First, they detained Kongou aboard her own ship. Second, they should be employing ambush tactics when going against Ars Nova... instead, they attacked her head on knowing that Ars Nova has a whopping five cores on board.
To Win Without Fighting: Iona attempts this several times with Kongou, but fails each time...until episode 12, when she successfully reaches Kongou, who was suffering from loneliness. She didn't want to kill her as she was forced to do with I-400 and I-402 earlier.
Villainous Rescue: In episode 12, I-401 is getting close to the US, only to get confronted with the massive US Fog Fleet in front of her, and an overzealous Kongou with an upgraded ship/death star. The latter then fires her Wave Motion Gun, only to wipe out the entire Fog Fleet in front of Iona. Several of the characters wonder if Kongou deliberately missed, but when they ask, she states in a monotone voice that it's to avoid corrupting them the same way Iona seems to have corrupted her fleet.
Violence is the Only Option: The attempted parley with Kongou only serves to convince Kongou to wipe Gunzou and his friends off the face of the planet. She continues to use this in her attempts to wipe them out, despite Iona's pleas and attempts to reach out.
Wham Episode: Episode 9 ends like this when having barely escaped from Kongou, Iona and Gunzou are ambushed by I-400 and I-402 just when they were proverbially catching a breath.
Episode 10 topped it by Takao sacrificing herself to restore Iona back, revealing that Maya is a fake simulacrum made by I-400 and I-402, and detaining Kongou of all people.
You Are Not Alone: Iona finally manages to reach Kongou, who up till this point had gone berserk and attempted to kill the former repeatedly. Once Iona manages to reach Kongou's core, she gives her a Cooldown Hug, and tells her she's not alone.