Is there one word for a harmful idea/plan? (All that comes to my mind is evil/wicked idea, but it's two words, and anyway it's actually a synonym for brilliant/cunning idea.)

The whole idea to slander John was... blah, blah...

Thank you guys. Plot, scheme, and intrigue will do. Scam is fine as well, only a bit too informal for the context I need.

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 ...

According to Merriam-Webster, you can use plot:

[ perhaps back-formation from complot ] : a secret plan for accomplishing a usually evil or unlawful end
_{from m-w.com}

Maybe "scam":

scam noun

: a fraudulent or deceptive act or operation

M-W gives the example:

Authorities said a parolee with a long history of identity theft and credit card scams masqueraded as a ride-share driver downtown and cleaned out the bank accounts of at least four people after gaining access to their bank cards.

So your sentence would be:

"The whole scam to slander John was..."

Maybe intrigue could work in your context -

Intrigue

: make secret plans to do something illicit or detrimental to someone.

(From Oxford)

Therefore,

The whole intrigue to slander John was... blah, blah...

There is also conspiracy -

Conspiracy

a secret plan by a group to do something unlawful or harmful.

(From Oxford)

Therefore,

The whole conspiracy to slander John was... blah, blah...