AL lore

In the future, a scientist called Dr Anzeel creates shipgirls using Wisdom Cubes and a part of a long sunken ship ((Operation Siren file datamine spoilers) the first of which being codenamed "Code G" or as we know her Enterprise, having been created from her anchor, which was stored in a museum at the time). The shipgirls are then developed by the different nations to keep up with the Eagle Union... But one day, another scientist, Dr Aoste ("The Magister" / "The Inquisitor") creates a replacement for the shipgirls, known as Antiochus. Dr Anzeel theorises that there's a higher state of emotion that exists called "Cognitive Awakening" and they come to the conclusion that they need a person with high wisdom cube compatibility to prove its existence (the commander)... Eventually a strange threat, giant steel Leviathans appear and attack humanity, leading to a war where humanity is short on supplies and most of the shipgirls are wiped out in a battle against them, with Antiochus not showing up in the battle to aid them. Anzeel was targeted by the Leviathans due to her being the one sending out communications to the world and in order to help the others escape, sacrificed herself by staying at NY City while the others fled but the commander turned back to save her and are implied to have died. Dr Aoste is given the surviving shipgirls and has them work together with Antiochus, sending them into the past to save the future (as seen in Fallen Wings, with him sending Observer Zero into the past), before sacrificing himself.

The remaining shipgirls (now known as the Ashes, having undergone a metamorphosis (hence the META name they go by in the game) that changes their Wisdom Cubes into "Origin Cubes") work together with Antiochus for a time but they decide to eventually take a different approach, leading to the two becoming enemies. Antiochus declares war on the humanity of the past, seeing conflict as an effective means of advancing technology faster (so they can fight off the Leviathans when the time comes) and they are given the name of "Sirens". After many years of battling the Sirens (for a while, without shipgirls (it's even implied their world's version of the Scapa Flow Scuttling was while they were still normal ships)), the world's nations come together to create Azur Lane, in the hopes of defeating the Sirens but eventually they have a falling out in regards to whether to use Siren Technology or not, resulting in the creation of the Crimson Axis, who believe that they can use Siren technology to defeat them... Which leads us to the main plot so here's some of the more lore important events since that post:

-Scherzo of Iron and Blood: Based on Bismarck's sinking, she is manipulated by Observer Alpha into using a black cube that causes her to go berserk. She manages to shake off the cube's effects and destroys a lot of the Sirens using her new powers but the Royal Navy still thinks she's being manipulated by the cube and sinks her.

-Ashen Simulacrum: The Ashes ships return again but this time they seem to be leading an attack on NY City. They set up a beacon in the Bermuda Triangle (they also set up one called the "Winter's Crown" in the event of the same name). These beacons serve as a space-time signpost and Observer Zero doesn't like that so she activates Arbiter: Empress III to stop them.

-Swirling Cherry Blossoms: Sakura Empire intend to perform a ceremony using a relic of their's called the Watatsumi, which can bring any ship, regardless of whether they had actual history or not, into existence via a ceremony when there's a Blood Moon. Iron Blood are also invited but suddenly, Ember shows up and attacks them, claiming that the Watatsumi gathers up negative emotions and will unleash them, causing a calamity, in addition to pointing out that it wasn't a part of the current timeline. She attempts to destroy it but it's been swapped for a fake, with Akagi giving the real one to Prinz Eugen.

-Microlayer Medley: A mist forms around a base nearby NY City and a group is dispatched to locate some missing shipgirls. During this Enterprise is shown visions of the Ashes timeline, where some of them group up (Enterprise, Takao, Hiryuu and Ark Royal) and start working together with Antiochus. The siren in charge of the experiment, Compiler shows these visions to Enterprise on purpose and Code G shows up to help them once again, revealing that the mist is called "Microlayer Medley", which disrupts communications signals.

-Northern Overture: The Northern Parliament requests help in taking down a Siren base. Afterwards, the commander is taken to their leader, Sovetsky Soyuz, who asks the commander if he would like to stay with them a little longer (worded as "joining the Northern Parliament" in EN instead)...

-Skybound Oratorio: Based on the Battle of Madagascar, the Royal Navy working together with Iris Libre try to recover a Basilica from Vichya Dominion but after arriving at the Basilica, an ancient prophecy starts occurring and Algerie uses a black cube. By destroying the Basilica, they're able to cancel the Black Cube's effects and bring Algerie to her senses.

-Aurora Noctis: The Eagle Union want the commander back from the Northern Parliament and while doing a convoy to the Northern Parliament, the Royal Navy agrees to bring him back with them. The Iron Blood attacks the Royal Navy but this turns out to all be a hologram, while the real fleet is targeting Scapa Flow and succeeds in blowing a hole in the anchorage at Scapa Flow, before travelling through to the NA Ocean.

-Dreamwaker's Butterfly: Shows the aftermath of Swirling Cherry Blossoms with Shinano being escorted. However an attack from Tester sinks her and she ends up in the Dream World, where she sees the future of various other timelines, including one where Akagi uses a Black Cube while fighting Ember, the aforementioned battle where most of the shipgirls sank in the Ashes / Siren timeline, a historical timeline that shows her historical sinking and one that's similar to the Home Port / Secretary Story one. She then wakes up, revealing that the sinking was also a dream of the future and defeats Tester. Akagi gets angry at Observer Alpha for attacking Shinano and asks for her end of the bargain for working with them (revealing the location of all Mirror Seas around the Sakura Empire) but doesn't get it. The Sirens teleport Compiler to the timeline, who is revealed to be an Arbiter and can now leave the hard work to the Arbiters, while they conduct their respective experiments.

-Khorovod of Dawn's Rime: Continues from Aurora Noctis, with the commander still in the Northern parliament. They take him to the Arcana Sanctum, an underwater mirror sea they took over back when they didn't have shipgirls and managed to acquire their technology from. The commander opens the path into the Sanctum with a Wisdom Cube, as they needed someone with high wisdom cube compatibility to progress further, revealing the City of Light, a dream world where they can view scenes from the Siren / Ashes timeline. In the end, Observer Zero, decides that Empress III should monitor the experiment while other Arbiters are sent after Code G.

-Operation Siren: Azur Lane and Crimson Axis have a ceasefire to deal with the Sirens who have taken over the entire North Atlantic in an attempt to capture Code G and the other Ashes ships. As they progress, they run into other Arbiters and their minions, the Enforcers. Plus the Commander's group runs into Hiryuu META, one of the Ashes ships along the way.

-Inverted Orthant: Iron Blood manages to take control of an old Mirror Sea and uses it as a training facility where they can hide their paper ships (created from Watatsumi). However Compiler hacks into the Mirror Sea's system and the Iron Blood pretends to play dumb by attacking their allies from all the other Axis factions (Carabiniere who's together with a group of Iron Blood ships to see the Mirror Sea as Sardegna's representative and Shoukaku / Zuikaku's group who were sent the long way around to regroup with the others during Operation Siren) until they manage to locate Compiler and destroy her.

Crimson Echoes - The event starts with the cutscene from Chapter 3 of Kaga sinking at Midway. While sinking she remembers the time of the Washington Naval Treaty (1920s in real life). The treaty's conditions ask for the navies to scale down their ships by a set amount and any ships that don't meet the conditions must be scrapped. The Sakura Empire holds a competition to see who will become next flagship: Amagi (Akagi's sister ship, when she was a battlecruiser) or Kaga (a Battleship at the time). Amagi wins and is scheduled to become next flagship but Kaga, realising that she's going to be scrapped decides to go out in battle and die there, leaving behind a letter as she goes into a Mirror Sea. Amagi finds her and saves her but Amagi ends up heavily damaged by the battle against a Siren carrier. At Amagi's request, Nagato holds another competition and Kaga comes out on top this time, with Kaga making a promise to protect the Sakura Empire and Akagi instead of her (so Kaga gets converted into a carrier instead of Amagi, while Amagi is scrapped). Back to present day, Kaga wakes up and notices that Akagi's personality is different and Akagi can't find the brooch Amagi gave her, with both heading to the "Sanctuary". This leads into Visitors Dyed in Red, with Akagi working together with the Sirens and planning to test the Fifth Carrier Division (Zuikaku and Shoukaku).

Visitors Dyed in Red - Zuikaku and Shoukaku find the key to the Sanctuary, which Akagi has set up as bait to lure them there. While fighting Lexington, they take cover in a storm but lose contact with each other. Following the storm, Zuikaku is seperated from the others in her fleet and arrives at the Sanctuary, where she finds that there's several Siren Pawns created of the other fleet members (fake versions of the ships). Akagi and Kaga watch Zuikaku's progress and intend to use a weapon made by someone referred to as "The Creator" but while Zuikaku's fighting the pawn of Shoukaku, she undergoes Cognitive Awakening, causing the original Shoukaku's personality to awaken in the pawn and protect Zuikaku from an attack. Akagi and Kaga send the test data to "The Creator" but Zuikaku's after them and defeats them with Akagi and Kaga escaping. The Sirens continue testing on the area.

The War God's Return (a mini event but it serves as a story between Visitors Dyed in Red and Ink Stained Steel Sakura) - After Visitors Dyed in Red, Zuikaku, Shoukaku and several others run into a Mirror Sea, where they encounter Tester Alpha, who was hoping to encounter Azur Lane, instead of Axis ships. Tester Alpha offers to let them join the Sirens but they refuse and Tester Alpha battles them but has all of their combat data from VDiR's situation, making her able to adapt to that combat data and easily defeat them. Thanks to scout planes that Shoukaku had sent prior, the former flagship Mikasa and her fleet show up and assist them, with Tester Alpha fleeing and opening an exit for all of them. The rest of the event is Mikasa being around the fleet and helping Zuikaku train.

Winter's Crown - Azur Lane's version of The Battle of the North Cape and Operation Tungsten. The Royal Navy sinks Scharnhorst and goes to attack Tirpitz, not sinking her just like they did in real life but Purifier interferes, destroying some of Victorious' planes and as a result Code G appears, creating a beacon called the Winter's Crown and telling off Purifier for dragging uninvolved shipgirls into her experiments. Purifier is defeated and she's sent to the Sacred Sakura at the end, which leads into Ink Stained.

Ink Stained Steel Sakura - Following The War God's Return, Zuikaku and the others arrive at the Sacred Sakura, a giant sakura tree where the nation got its name from, where they find that the area is surrounded by sirens. They intend to wake up Nagato, who was put into a deep sleep after the war began to hear from the spirits inside of the sakura tree and learn from them (the sirens surrounding the tree actually being there to guard the tree, even back when she was sealed away). Akagi and Kaga watch over this as well but decide not to battle them, since Nagato is just a single battleship and its revealed that their goal is to get someone who can oppose the sirens and change history. The fleet encounters Kawakaze, Nagato's bodyguard, who made a promise to not let anyone wake up Nagato but after a duel, she lets them go to Nagato. The sirens start attacking the Sacred Sakura and Nagato awakens but she tries to go back to the tree, when Mikasa convinces her to stay awake and have a duel with them, alongside her sister, Mutsu who is also near the Sacred Sakura. Purifier shows up and informs them that "When the Winter's Crown is donned, when the Steel Sakura blooms... the Souls' Transmigration will begin... and start the Final Prelude". They defeat Purifier and Akagi acquires a weapon that will help them defeat not only the Sirens but Azur Lane as well. The story then continues in Swirling Cherry Blossoms.

Divergent Chessboard - Set in a Mirror Sea, this shows us the events that lead up to a pawn of Tirpitz Cognitive Awakening during many tests the sirens conduct. During the test, a couple of Iron Blood submarines end up in the Mirror Sea and try to escape to inform the Commander of what's going on in the Mirror Sea (seemingly implying it's the latest event in the timeline at the moment).

Fallen Wings - The Eagle Union go to investigate a Siren Research Facility that's been destroyed recently. This all leads up to them encountering Code G, who rescues them but quickly leaves before she can answer their questions. The event has various logs they find of the Ashes timeline.

The other two major events, Iris of Light and Dark / Empyreal Tragicomedy are more historically based (Operation Catapult / Torch and the Battle of Taranto / Cape Matapan respectively) and don't really add much to the plot, aside from explaining why Vichya hates the Royal Navy or why Sardegna wants to re-join Azur Lane, despite still being a Crimson Axis faction / their reason for being in the Axis.