Charles Darwin's notebooks contain an important sketch of a tree, the forerunner of his famous theory of evolution, written in the 1830s. They were last seen in Cambridge University's library in 2000 when they were taken out for pictures.
Librarians discovered the notebooks were missing two months later, presumed lost among millions of items. Finally, when the notebooks still hadn't turned up twenty years later, the library admitted that they were probably stolen.
Amazingly, a pink bag containing the notebooks was left in a public part of the library with no surveillance, near the librarian's office. An accompanying note wished the librarian a Happy Easter.
Turning this into a story, what if we write a book about a reverse heist?
The story starts with a person accepting a pink bag with explicit instructions about where to leave it in the library.
Then, we flashback to a young man finding the notebooks in a random part of the library. Realizing they were important, priceless documents, he holds them for safekeeping. He'll bring them back as soon as the library realizes they're gone.
It's a quick lesson that takes twenty years.
We see him keeping the documents hidden from his girlfriend, who becomes his wife. He hides it from his kids and almost loses them during a move.
We see him planning the reverse heist during the chapters about the passing years. He finds a random person with instructions for finding another random person, ensuring that even if the library discovers who dropped the bag, they can't trace it back to him. We see him scouting the library, finding where the person could sneak in and place the bag with minimal chance of discovery.
Cut back to a chapter when he finds out that the library finally admits the books were stolen, and the plans for the heist begin.
The final chapters outline the reverse heist, showing the person who kept the books all those years getting his affairs for his wife and kids if he's hauled off to jail for the theft. Then, on the day of the drop, he goes to the library for a bit of light reading, watches the pink bag deposited outside of the librarian's office, and knocks on her door to report the item left behind before walking away.