The one single fact which can give significance to an entire story is sometimes called a ‘significant detail’. This can convince the reader that the story is plausible, could actually happen.
Show rather than tell the significant detail in vivid and realistic detail to spark in the reader’s imagination, the cinematic or fictive dream.
The last sentence is comforting. Your wildest ideas are fine, so long as you can find that one fact that makes them believable. You can do anything!