noun
machination
mash-ih-NAY-shun/ˌmæʃɪˈneɪʃn/
A secret, crafty scheme intended to cause harm.
The secret envelope exchange forms part of the pair's underhanded machination.

Etymology
Latin machinari covered both skillful designing and plotting. A machination is thus a contrivance in the mental rather than mechanical sense—a carefully devised scheme, with the underhanded meaning already present in its ancestry.
More examples
- She uncovers a machination designed to force her resignation.
- Their financial machinations leave hundreds of retirees without savings.
- The investigation exposes the rival company's machinations.
Forms
machination · machinations