Your heroine receives a piece of paper with a cryptic code on it. As she deciphers the code, it leads her to….?
Who gave her the piece of paper? Describe the situation she’s in when she receives/finds the paper?
How does she decipher it?
What does it say?
Where does it lead her?
This could be a suspense for sure. It could also be contemporary. And if historical is your genre, have fun with it. There’s nothing better than an intelligent, curious, fearless heroine!