To devise is to figure out a plan. Men twirling long mustaches might devise a plan to tie someone to the railroad tracks.
To devise a plan is more than just making one; there's a lot of figuring out to do, such as: "Phoebe devised a bus route that picked up kids in alphabetical order." There's a method to the madness of devising. The chemistry class devised a plan to make the fire alarm go off. Devise can be positive, too, meaning to find a creative solution: "The design team devised a plan to make the trailer home look like a medieval castle."