Striker Liam Dickinson - making his Seasiders debut after joining them from Derby on loan in the week - was introduced as a 76th-minute substitute and scored with his first touch in the 79th minute and then claimed the 81st minute winner to end the visitors' woe
Argyle had taken the lead in the 67th minute when on-loan Blackburn forward Paul Gallagher fired home from 20 yards.
Gallagher's intuitive strike, from substitute Steve MacLean's pass, was his seventh goal for the Greens and looked set to send Paul Sturrock's side on their way to victory.
But Grayson - and super-sub Dickinson - had other ideas.
"I thought we were the best team throughout the course of the game," Grayson said.
"I thought it was against the run of play that we went 1-0 down and the run that we have been on, you obviously think 'is it going to be another defeat'?
"But we have a lot of faith and confidence in the players and they dug in there and showed the proper desire to get back into the game. They didn't fold, they kept going and got the result.
"We had to make the decision to bring the three subs on and it's one way (for Dickinson) to make an impression, scoring with your first kick and then obviously two minutes later as well.
"Liam's here to make an impression and has obviously started exceptionally well."
Plymouth boss Sturrock believed his side paid for failing to add to their opener.
He admitted: "When we went 1-0 up it was important that we went on to get a second goal. That's what I wanted us to do but unfortunately we dropped 10 yards deeper and invited them on to us.
"If we had scored a second it would have made a big difference but we didn't.
"As far as I am concerned, though, the game was lost in the first 45 minutes and not the last 10.
"It's very frustrating but we seem to have a mental block.
"If it was physical I could do something about it but we seem to think because we played well to beat Cardiff and then got a draw against Southampton, then that's it.
"But I warned them it wouldn't be like that and Blackpool proved it."
Sporting Life