The Pilgrims twice squandered a two-goal lead and, in the end, were lucky to hang on for even a point as Watford, reshaped and re-organised, threatened to steal the match.
Michael Evans headed in a cross by Akos Buzsaky from close range and Tony Capaldi stroked home a shot from 20 yards to give Argyle a 2-0 lead by the 12th minute and, even though Marlon King pulled one back before the break, the Pilgrims immediately restored their two-goal advantage in the 43rd minute through captain Paul Wotton.
However, Ashley Young struck twice in the early stages of the second period, the first direct from a free-kick, after which it was the visitors who came closest to taking all the points when King screwed a shot wide of the post.