If the plot is "way too easy to scope out", then perhaps your mastermind isn't as smart as he thought he was. This happens in real life a lot - search for "dumb criminals", or just look at famous examples in history. *Why would anyone be so foolish as to invade Russia in wintertime?* Yet it has been tried more than once, by people who "should" have "known better".

No one, not even a villain, can plug up *every* weakness. They have weaknesses, vices, or distractions that can provide an avenue for defeat - *if* the PC's detect them. Detecting them is part of the fun!

If your players' characters are smart enough to figure this one out, then you should give it to them. That's part of the game - allowing them to come up on top by smart thinking and strategizing. This applies especially if your players prefer [combat as war](https://rpg.stackexchange.com/questions/76399/how-can-i-encourage-combat-as-war-style-play-in-my-campaign), where "even" fights are not as emphasized.

Even once your PC's "know", that isn't the end of the story! How do they convince others (e.g. the king, or the Senate) of the mastermind's plot? Can they *stop* it? Can they trick the mastermind into not realizing that he has been exposed? Maybe the PC's "know" the truth, but local authorities are skeptical and unwilling to send in the Army to arrest the mastermind unless further evidence of the mastermind's plot is presented to them. Do your PC's go out in search of that evidence, or do they try to take down the mastermind alone? Maybe local authorities are in cahoots with or under the control of the mastermind already, so the PC's end up getting arrested themselves on trumped-up charges and thrown in prison.