The protagonist, having apparently died in some supernatural event while sitting in Classical Literature class, wakes to find herself trapped in an egg. Freeing herself, she learns that she is a spider, and soon she finds that she is in a fantasy world, with levels and skills and all the trappings of an RPG.
So she does the natural thing, and exploits the system as much as possible in order to grow stronger–the weak, she has seen, are doomed to be eaten.
Interspersed with the spider's story is the story are scenes featuring Shunsuke Yamada, who has been reborn as Schlain Zagan Analeit, called Shun, and Kanata Ooshima, who has been reborn as Karnatia Seri Anabald, called Katia. The former is a prince and the latter the daughter of a duke. These scenes are set a decade or so in, while the spider's scenes begin, one assumes, begin as soon as everyone is reborn (since she starts by breaking out of an egg). Anyway, there's a time gap.
At this time, the Demon Lord's army is preparing to attack. Shun's brother Julius is the hero, so he should be instrumental in the war.
This volume covers through the end of Chapter 14-4 of the manga.
The novel ends with the protagonist defeating an army of anogratches and bagragratches (monkey monsters), and learning that she can evolve, now that she's reached level 10, into either a taratect or a small poison taratect.
The final chapter shows Julius entering the Great Elroe Labyrinth to deal with a unique taratect that has been spotted. Could it be our protagonist?
Who is the protagonist?
The light novel begins with the class register, and the protagonist isn't named, so clearly we must wonder: which one is the protagonist? She is probably the one we see in a flashback called "Rihoko", short for "Real Horror", who is described as "super-creepy, all skin and bones, with a pale and permanently dour face". She is apparently not well-liked. Her real name isn't given, though, so it's not clear which of the people on the class register she is.
That said, the protagonist does say a couple of alarming things: "Maybe I’m not the real me at all, just some total stranger who happens to have the memories of another person." and "It’s not like there’s any way for me to know for sure. How can I prove I’m not who I think I am, anyway?" Foreshadowing, perhaps?
The full class register is here, with my notes about the characters' identities–so it'll be a spoiler, unless the identities of the students turns out not to matter.