I need help to think of a mystery plot (HELP!!)?

Question:

I need help to think of a mystery plot (HELP!!)?

(BOOK IS 4 FUN)
The two main character are going to be 2 guys and 2 girls (ages- 20-21) I want it to be mystery. And I don`t want any of the 4 main characters falling in love.
I will read anything (It may spark me an idea)
Thanx :)

Additional Details

1 week ago
I have already wrote 5 other books with these same characters.
This is #6 in a series

Best Answer - Chosen by Asker: 1 week ago
I have already wrote 5 other books with these same characters.
This is #6 in a series