The missing document is the MacGuffin [məˈɡʌfɪn] that brings the two main characters together, but the real story centers on their tumultuous relationship.

MacGuffin [məˈɡʌfɪn]
noun

A MacGuffin is an object, event, or character in a film or story that serves to set and keep the plot in motion despite usually lacking intrinsic importance.

The first person to use MacGuffin as a word for a plot device was Alfred Hitchcock.

English