Despite going down to ten men against the team with the best away record in the Championship, Argyle managed to hold on for a point that keeps them in the top-six.
Appropriately for the weekend, they went ahead in an edgy, niggly game, through Jermaine Easter, who notched his sixth goal of the season.
The Wales international latched on to strike partner Rory Fallon's flicked header to take the ball around Leigh Bromby and place a shot beyond Richard Lee.
The lead lasted just three minutes though as Watford levelled through Lee Williamson's expertly flighted long-range free-kick, awarded for a foul by Fallon.
Argyle midfielder Chris Clark wasted a good chance to put the Pilgrims back in front before the interval when he failed to convert Paul Connolly's cross.
After the break, Watford defender Dan Shittu went close with a drive from the edge of the box before Sadler and Halmosi were booked after a fairly innocuous clash.
When Halmosi tripped Sadler a few minutes later, that ended his participation, and made Watford favourites to take the three points.
Jobi McAnuff wasted their best opportunity when he sneaked into space on the left-hand side of the Argyle defence and curled a shot just wide of the post with Pilgrims goalkeeper Luke McCormick well beaten.
However, ten-man Argyle held on, with substitute Jim Paterson kneeing a powerful trademark Shittu header off the line, and nearly sneaked the win themselves when captain Paul Wotton drilled a free-kick against the foot of Lee's post.