Is this idea any good?

[Aloïs]

This is a pretty standard idea. It's strictly speaking barely even a plot. It's more like a premise idea. What's going to decide if it's a "good" plot, or a "average" one will be determined on several factors following the simple premise of a simple basement.

What exactly has happened? Is it something from super long ago? Is it something that is happening now?

You as the creator have to weave together a logical, and intriguing story from the plot aspect of the "secret basement". Remember, even though intriguing is important, logical must always come first as without that no matter how interesting the story is, the basics of the plot will fall apart because of plot holes.

You are right. I'll try to mainly focus on how logical it is.