Every Pokemon Mystery Dungeon game at their core follows a very simple set of story beats,
all of which begin with a very simple human -> Pokemon isekai plot. In EOS, you begin the game by answering a personality quiz
which assigns you one of the 21 playable Pokemon. After that, you're immediately thrown into a cutscene, and wind up unconcious on a beach.
From there, you meet up with another Pokemon, who you soon team up with to create an exploration team in order to explore one of the many dungeons in the game.
These are the things the game is built around. Mystery Dungeon are a collection of randomly generated floors, each containing items, enemies, traps, and a
staircase which you must use to progress. You and your partner explore dozens of these throughout the story, and if either of you faints, you lose all your items, and are sent back to the beginning. The gameplay is similar to roguelikes in this, and can get frustrating at times (ex. monster houses), but
I've always found it to be a very rewarding experience, especially once you master all of the game's mechanics.
As much as I love this game, I'm always very hesitant to recommend it to anyone. The dungeons can be frustrating at times, both due to length and difficulty, and it IS written to be an all-ages DS game from 2009. However, I still think it holds up really well today. The story touches on some stuff that isn't really seen often in games like these (the final special episode, especially), and really nails a lot of key beats where it counts to push itself as far as it can. For a game with the premise "do our uquiz and we'll put your self-insert into a dungeon-crawler", it does some really fantastic things.
Web-hosted version of the in-game quiz
Rom editor (Change your starter, etc)
Mystery Dungeon sprite repo