A "devious plan" refers to a cunning or deceitful scheme in which someone tries to achieve their goals in a sneaky or manipulative way. It suggests that the person's intentions are not straightforward or honest. Full definition
Amanda is full of devious plans and dark ideas; she blackmails a drug dealer Tim (the late, Anton Yelchin, of Star Trek fame who passed away shortly after making this film) to do the deed. (sarahsbackstagepass.com)
In light of the recent Windows 10 announcement I realize that Microsoft has a much more devious plan in mind, the assimilation of the entire world... Maybe not the entire world, but at least those pesky PC users who have proven to be elusive when it comes to embracing Microsoft's online products. (relevant-gaming.com)
Or maybe this is all some cartoonishly devious plan from Cuomo - stan to actually get the votes of progressives who hate him, though we doubt it. (thealbanyproject.kinja.com)