The 17-year-old youth product equalised with a superb left-foot shot on the spin from the edge of the box in the League Two clash on Boxing Day.
Seconds earlier, United keeper Michael Poke did well to flap out Robbie Williams' rising drive but Harvey was not to be denied on his first home start for his hometown team.
Wind-assisted Argyle had far better of the first spell with left-back Williams firing a 12th-minute warning shot just wide from the left angle of the box.
As the second half sprung into life, Argyle keeper Rene Gilmartin did well to keep out Kevin Nicholson's thumping 25-yard free kick but could not stop the ball spilling out for a corner.
And when Nicholson swung the ball in, Torquay central defender Aaron Downes roofed in the 78th minute opener.
Downes headed the ball down and then followed up with a thumping right-foot finish which looked set to seal the Devon derby in front of United's 1,525 visiting fans before the late twist.