Utah will take on Drexel in the championship game.
Brittany Chambers scored on a layup for Kansas State with 2:14 to go in the second half, forging a 39-39 tie. Neither side could score again in regulation and Utah eventually began to dominate in the overtime.
Utah ended the extra period on a 13-5 run with Plouffe scoring six points in the game-closing surge.
Plouffe finished with 24 points to lead Utah (23-13), which wound up sixth in the Pacific-12 Conference regular season race.
The Wildcats (19-18) were paced by Chambers with 16 points.