Nick Chadwick's close-range strike at the start of the second half had looked good enough to earn Argyle only their second away victory of the season but the on-loan Nottingham Forest striker, who will be joining the Hornets on a permanent basis in January, struck home a terrific right-footed half-volley from the edge of the area to preserve his side's ten-game unbeaten League run.
Chadwick's goal enlivened a match that, in the first half, had largely been something of a tactical chess match, with Watford attempting to break through the visitors' disciplined and well-organised defence.
Chances were few, although Plymouth could have taken the lead as early as the fourth minute when, following a surging Jason Jarrett run, David Norris cut inside but saw his left-footed strike narrowly deflected wide.
The Hornets then had most of the play but were unable to create an opportunity until shortly before the half-hour when King glanced an Anthony McNamee free-kick goalwards.
A combination of Elliott Ward and Romain Larrieu enabled Plymouth to partially clear their lines but Gavin Mahon's piledriver looked to be going in until it struck the keeper painfully in the face.
But the complexion of the match changed within two minutes of the start of the second half when Paul Wotton headed a corner from the left goalwards and Chadwick spun in the six-yard box to squeeze the ball beneath Ben Foster.
It looked like it wasn't going to be Watford's day when King missed a great opportunity to equalise on the hour. Ward failed to deal with Matthew Spring's pass forward leaving the striker clear on goal but he dragged his short so horribly wide it went out for a throw.
The home side threw defender Clarke Carlisle up front to partner makeshift striker Jay DeMerit as their search for a goal became increasingly more desperate, but the visitors should have wrapped up the points with seven minutes remaining when Norris slipped in Tony Capaldi but he fired badly over.
And Plymouth were to rue that miss two minutes into injury-time when some excellent McNamee wing play saw him lay the ball off to Malky Mackay in the area.
The defender, who had also been pushed up front by this stage, delightfully cushioned the ball back towards the edge of the area where King struck a fine effort past the static Larrieu.