Writer's choice: The Genetic Inheritance Affair (1-4)
This story about Agnes Dabree's attempt to create a perfect race of supermen to get revenge on her tormentors started out to be a fairly humorous affair, and could have finished after Part 2, but it seemed to need to go further and darker so there are four parts.
https://archiveofourown.org/series/955545
Mr Waverly telling Illya and Napoleon what THRUSH has in mind for them