Does the whole film take place on the bottom floor of a skyscraper? If so, how is the floor divided up? Can they go outside?
At 61 words this is a little long for a logline. Ideally, it should be under 40 - the shorter the better. I appreciate, however, that there's a bit of world building you need to do and an MPR.
I'm not sure the bit about the bottom floor of the skyscraper is necessary. I think the story is the same either way - although, if the whole thing is shot in that one location... it's interesting. I could be swayed either way.
I would move a few things around and trim it down a little:
In a world devastated by a plague, when a power-hungry overlord begins a genocidal mission over the depleting resources, a cunning thief must lead a uniquely skilled team of people to assassinate him. ?(33 words)
I've removed the MPR for the moment as I'm not sure what the thief's goal would be after they discover this. I'm not even sure it would make any difference to the overlord's desire for resources so the goal of killing him would still be there. There's no information that tells us the plague has disappeared so, as far as the reader goes, it's still there anyway.
"discovers that the plague never left and that could cost him everything" - he's set to lose everything anyway if the overlord gets his way. This is another reason I've left it out. I think having the plague a constant threat adds a level of tension that will trickle in throughout.
I would keep the bit about the bottom floor of the skyscraper IF going outside is dangerous AND in order to achieve their goal the thief and crew must go outside. It must be relevant to the story you are trying to tell.
Cool idea! Looking forward to seeing where this goes.
Hope this helps.