The novel starts with Touma getting sucked into another world by his toilet. He’s quickly joined by Index (who was in the middle of changing) and Mikoto. Imminent punishment on Touma is averted when they notice a dark dragon fighting Baby Magnum nearby. Several other characters are also witnessing the fight. The characters transported to the world are:
- Toaru Majutsu no Index: Kamijou Touma, Index Librorum Prohibitorum and Misaka Mikoto.
- Heavy Object: Quenser Barbotage, Heivia Winchell and Milinda Brantini (Princess) in Baby Magnum.
- The Zashiki Warashi of Intellectual Village: Jinnai Shinobu, Yukari (Zashiki Warashi) and Yuki Onna.
- A Simple Series: Anzai Kyousuke, Higashikawa Mamoru and Bunny Girl Karen.
- Killer Princess and Deep End: Kyouichirou, Murder Princess Satsuki.
One of Baby Magnum’s shots bounces off the dragon and almost kills Quenser and Heivia, but Yukari stops the shell with one hand (Youkai hax
). When the dragon continues to attack Yuki Onna freezes it. The dragon tries to rise again but is suddenly sniped by a giant beam. Enter Waltraute and Jack Elvan (The Circumstances Leading to Waltraute’s Marriage).
This is all adapted in the first chapter of the crossover manga. From now on it’s all Google Translate so expect errors aplenty. Feel free to correct mistakes.
The world they are in is Waltraute's world. After the fight with the dragon the heroes decide to cooperate and look for a way home. Something about them using Skíðblaðnir (Freyr’s boat and the myth used by New Light in Index to make their magic suitcases) I think. Waltraute brings them to Midgard. In the journey the female’s clothes are destroyed which forces them to wear the erotic outfits you can see in the color illustrations.
They stay at an inn in Midgard and decide to wait while Waltraute looks for a way for them to return to their world. But they need to earn money to pay their expenses while they wait. A suspicious half-dead beauty appears and an army of giant (20m) ice soldiers attacks the town. The group collaborates to defeat them and they earn a reward.
The suspicious girl is Hel from Waltraute’s world (she appeared at the end of chapter 1 in Waltraute’s novel and is the new girl in the manga group shot). She’s trying to get their souls because they are rare souls from another world. She was the one who summoned the giant ice soldiers and Níðhöggr (the dragon at the start).
Not sure about the next part, probably Google Translate messing something: Hel was hit by the dragon’s breath and ended crushed into a wall. Touma tells her the stories of "Momotaro", "Princess Kaguya", "Urashima Taro" and she cries. Anyway, she’s interacts with the heroes and learns a bit about them and their worlds.
In Waltraute’s world the souls of excellent warriors are needed for Ragnarok so by the rules created by Odin those who have not killed anyone are sent to the underworld (Hel).
Something about Shinobu being frozen, but I can’t make out what it means.
The mastermind who brought the group here is Loki, who forced the branches of Yggdrasil to reach the other worlds. The group needs to use Yggdrasil to go back to their worlds.
Hel is the one who takes the souls of the elderly, children, those who die of sickness; basically anyone who doesn’t die killing enemies in the battlefield. Murderers "ascend to heaven" (as close as it gets in Norse myth) while innocents fall to the underworld. She realizes how warped these rules are because of the contact with people from other worlds. Hel decides to rescue all the souls in the underworld, but she might die in the process and cause a calamity worse than Ragnarok. A jet-black huge tree appears. Jack wants to save Hel and the rest of the group decides to support Jack.
Hel summons shadows of people from the other series to fight the protagonists using the black tree. These are:
- The 100m Second Generation Object: Rush and its pilot Ohoho (Heavy Object)
- The freelancer youkai-buster agent: Hishigami Mai (Zashiki Warashi)
- The serial killer who only needs a stun gun to create a charred corpse: “Death by Electricity” Shizuna (Killer Princess)
- The existence who symbolizes the Absurd: "White Girl" Objective Alice (A Simple Series)
- The strongest Level 5, Academy City’s #1: Accelerator (Index)
Waltraute faces the eight other Valkyrie sisters, who are aiming to kill Hel to stop the catastrophe.
Fenrir starts to rage and try to break Gleipnir (the chain binding him) because he wants to save his little sister. Shinobu pulls a Tyr and puts his hand inside his mouth and promises him he will carry his anger and save Hel in his place. Fenrir stops raging and leaves the rescue of his sister to the hero group.
Thanks to this action the god of war and victory Tyr is willing to cooperate with the heroes and gives them the buff “Absolute Certain Victory”. The heroes start to push the shadows back.
But then the black tree extends a branch to a certain world. The ultimate goal of Loki is revealed: he wants to extend a branch to a “primordial world”. The world is called「○○○○○○○○の○」. Loki needs this world to fulfill his desire. However something that couldn’t be predicted by Loki happens. The new world connected is not the one Loki seeks.
The new world is the one from “The Unexplored Summon://Blood Sign”. The shadows of Shiroyama Kyousuke and Meinokawa Higan appear. Kyousuke wastes no time and summons the strongest “unexplored class” summon: The “White Queen” who Wields the Sword of Unsullied Truth. The hero group gets stomped hard.
From Asgard Odin decides to wipe out the tree using the God Spear Gungnir, which would also wipe out the hero group. Fortunately Othinus intervenes and stops the use of Gungnir, but only temporarily.
Jack “suicides” to go to the underworld and asks help from the god of light Baldr (currently dead after being killed by Loki as mentioned in Waltraute’s novel). Baldr uses his power of light to protect the heroes. Thanks to that Gungnir only wipes out the tree and the shadows. Jack talks some sense into Hel and the incident is resolved.
However, when they open the gates to their worlds a large amount of seawater appears and threatens to sink Waltraute’s world. This is related to the sea goddess Rán who is trying to evolve into “a god of death who protects all the seas” using the "Absolute Capacity Evolution Plan (level 6 shift)" - 「絶対能力進化計画（レベル６シフト）」 (
Kamachi
). Joining her is her husband Ægir.
The heroes stop the tragedy and Index returns everyone back to their own worlds. Everyone except Touma, of course. His return is stopped by Imagine Breaker and he gets trapped in the Void between worlds. Just as he starts despairing about being stuck in an infinite world again Othinus brings him back to Index world.
Earlier spoilers about the shadow fights not present in the summary:
- Accel faces Touma for about the entire fight.
- Alice has the ability to make all attacks become ineffective, so Satsuki has to change her murder method each time to stall her.
- The White Queen doesn’t take the battle seriously and just plays around.
- The White Queen noted that even Othinus in her Full Power mode is still nothing compared to her, the strongest being in the unexplored class.
After the summary there are also more details about each series' characters:
As expected Touma acts as "leader" of the group (as much as a group formed by those people can have a leader
) and he has to suffer being lumped with super perverts like Quenser and Heivia by the girls of the group. Mikoto was one of the most useful combatants and apparently did well in the fight against the Object. Index was also very useful thanks to her analysis of Norse myth and Mai's powers. Apparently when Othinus appeared to stop Odin she had a chat with Frigg and they knew each other!
Quenser and Heivia were the main culprits of sexual harassment towards the girls and were related to the destruction of the girl's clothes. They collaborated with Mikoto against Mai doing things like throwing Hand Axe to her face. Of course the Princess had the most firepower of anyone in the hero team while in Baby Magnum but as a result of spending so much time inside she doesn't interact much with others.
Mamoru is still the “Ever-Victorious Challenger” and Karen the “Unbeatable Emperor” but Kyousuke (the one from A Simple Series) doesn’t have a supernatural ability and is the most ordinary person in the hero group. While Mamoru is someone who confronts the Absurd Kyousuke is someone who observes it.
About Killer Princess: in that world there are 72 murderers who control causes of death like burning, drowning, electrocution, etc. These aren’t “fantasy abilities” or “advanced science”, they kill people using normal tricks. The blogger compares it to Zenjiro Kasai from Majin Tantei Nogami Neuro.
The 「○○○○○○○○の○」 primordial world Loki was seeking is probably シュレディンガーの街 a.k.a. Schrodinger Road. This was Kamachi’s first work, the story he wrote for the 9th Dengeki Game Novel Prize that didn’t pass the third selection stage but caught the attention of Dengeki’s editor and led to him being a professional writer.
I'm quite excited to see Kamachi's protagonists interact with each other so as long as the crossover has its share of comedy and banter between them I'll enjoy it!
) Gankutsuou; Railgun S; Little Witch Academia (one of the most philosophically interesting/deep shows that I've seen, while also being the single most feel good of feel good shows that I have ever seen; literally the weirdest combination ever); Kill la Kill (because it saved anime 
than touma gets struck and a deus-ex-othinus saves him 
(LO is danger? 
