Watford were the better side for the majority of a tense affair, with a bobbly surface preventing either side from producing much fluent football.
They got their reward five minutes after the interval when on-loan Icelandic striker Helguson pounced from close range for his 10th goal since joining from QPR.
The result lifted the Hornets three points clear of the relegation places but leaves Plymouth five points adrift of safety with only three games remaining, with promoted Newcastle next up.
The kick-off was delayed by more than five minutes after referee Colin Webster pulled a calf muscle during the warm-up and was replaced by Warren Atkin.
Plymouth midfielder Chris Clark was the first to threaten but failed to hit the target with his speculative shot and Ross Jenkins blazed over at the other end during a scrappy and nervous opening.
Watford began to get on top and had the opportunity to threaten from a free-kick just outside the area but Damien Johnson's setpiece was headed wide by Helguson.
Poor marking from a corner allowed Plymouth midfielder Yannick Bolasie to find space at the near post but his header failed to trouble Watford goalkeeper Scott Loach.
The deadlock was almost broken around the half-hour mark when the ball fell to Hornets striker Danny Graham but he snatched at the ball and his miscued shot bounced over Plymouth goalkeeper David Stockdale and struck the outside of the post before being cleared to safety.
The home side went close again late in the first half when Don Cowie's flick rolled agonisingly wide of the far post after good work by Helguson but the half finished goalless.
Watford's pressure continued after the break and Eustace's header was saved by Stockdale before Helguson should have done better when the ball arrived at his feet quickly and he stabbed it over from eight yards out.
He made amends just two minutes later though as the deadlock was finally broken. Cowie and Graham did well down the right and the former's cross was met by Eustace, who challenged Stockdale in the air and Helguson was left with a simple tap-in from close range.
Plymouth's Jamie Mackie was then booked after he went in late on Dale Bennett before Helguson headed over.
The visitors offered little in reply despite going a goal behind and Paul Mariner made a double substitution in the 67th minute in a bid to provide his side with a creative spark.
It proved in vain though and Watford comfortably held on, with Loach rarely called into action.