I favor scheme. While it can refer to plans more generally, the connotation is commonly negative. From the Oxford English Dictionary, bold for emphasis:
5 b. Hence, A plan of action devised in order to attain some end; a purpose together with a system of measures contrived for its accomplishment; a project, enterprise. Often with unfavourable notion, a self-seeking or an underhand project, a plot (cf. scheme v., scheming adj.), or a visionary or foolish project. Phrase, to lay a scheme.
And Cambridge Dictionary:
A scheme is also a secret and dishonest plan:
Villains have schemes. Con artists use Ponzi schemes or Pyramid schemes. A "scheme to slander" appears in many search results. Hence:
The whole scheme to slander John was ...