Etrian Odyssey Untold: The Millennium Girl is the better version of the original Etrian Odyssey. It has a more linear story mode, with well developed characters and an extra dungeon, and it tries to help new players by adding an easy mode and other qualities of life. But, if you want, you can also play the old, hardcore version of the game. So, it helps new players without ruining the fun to the old ones.
The plot is good, I really liked the story mode. I think this is, by far, one of the best games by Atlus. But maybe Etrian Odyssey V is better. I should replay that.