Midfielder Chaplow - recalled to the Preston starting line-up after missing the previous week's home defeat by Barnsley - struck in the fourth of four minutes of second-half injury time to complete a fine comeback from North End, who were 2-0 down with 25 minutes to play.
Argyle striker Jamie Mackie celebrated his full home debut by scoring with a quality strike in the 12th minute after latching onto a breakdown 35 yards from goal and sending in a dipping shot that sailed in over Preston goalkeeper Andy Lonergan's head.
Although the shot was deflected, it was a remarkable way to mark a first Home Park start.
Argyle nearly took a two-goal lead into the interval when Rory Fallon nodded down defender Jim Paterson's left-wing cross into the path of Hungarian international winger Peter Halmosi, who showed excellent technique to bring the ball down for a left-foot shot but pulled it wide.
The lead was doubled midway through the second half when Argyle captain Paul Wotton, who had been left out the squad altogether in the previous game, netted a penalty for his first goal since October 2006.
Preston conceded the spot-kick when Youl Mawene wrestled Krisztian Timar to the ground in front of referee Andy Penn and Wotton, who missed a season-and-a-half through a knee injury, blasted the ball high and to Lonergan's left.
Preston brought on Simon Whaley straight away and immediately reduced the arrears when Neil Mellor got on the end of a punt that Timar and Russell Anderson had both left to goalkeeper Luke McCormick.
It looked like being a mere consolation for the visitors until Mellor capitalised on Paul Connolly's slip to tee up Chaplow for the dramatic conclusion.