The Preston midfielder slid the ball in from close range to cap a fine late comeback from North End, who at 2-0 down looked dead and buried at Home Park.
Jamie Mackie marked his return to the side with a goal in in the 12th minute with a shot from well outside the penalty area which flew into the net via a deflection.
And Argyle doubled their lead from the spot, Paul Wotton firing in after Krisztian Timar was upended in the area.
But Preston striker Neil Mellor started the comeback with a strike from eight yards following Karl Hawley’s assist.
And Chaplow struck the equaliser late on after Mellor’s pass.