Paul Gallagher's double lifted Plymouth above Watford in the battle to avoid relegation from the Championship.
Ashley Barnes hit the woodwork twice for Argyle before Gavin Hoyte hauled down Alan Judge and Gallagher scored from the penalty spot.
Tommy Smith took advantage of a mix-up between Argyle duo Marcel Seip and Craig Cathcart to level, nipping in and rounding keeper Romain Larrieu.
But Gallagher dinked the ball over Scott Loach to win it for Plymouth.
Plymouth manager Paul Sturrock:
"What really pleased me is when Watford got it back to 1-1 we showed the character to go on and win the game. That may not have happened a few weeks ago.
"It's been long overdue but I think we were worth it tonight. I did not think we played as well in the second half but you have to give credit to Watford, they improved in the second half.
"Now we have got a biggie at home to Reading on Saturday, and they won tonight and the teams down with us - like Southampton and Norwich - have won so there's still plenty to play for and I won't be happy until we are past the 50-point mark by some way."
Watford manager Brendan Rodgers:
"We just never got going first half and didn't get high enough up the pitch.
"We made two mistakes and to be fair they are probably the first mistakes we have made in about five games.
"I was happy with the second half, with their character and it was a terrific goal by Tommy Smith."
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