The winger's fine run and finish with just two minutes remaining saw Graham Westley's men to victory in a game which looked to be heading for a draw until the late, but great intervention.
Stevenage made two changes from the team that picked up a point at Wycombe with Ben Kennedy replacing the suspended Chris Beardsley and Charlie Lee returning to the starting eleven at Dave Martin’s expense.
Former loanee Ryan Brunt received a warm welcome on his return to The Lamex and he caused Boro problems as early as the fourth minute, flicking the ball on for Bobby Reid in the area only for Dembele to slide in with a perfectly timed challenge. The visitors appealed for a penalty but claims were dismissed by referee Haines.
Stevenage pushed forward but weren’t making much stick, with Plymouth happy to enjoy the bulk of the first half possession. They came close to taking the lead just past the quarter hour when trickery just inside the box by Lewis Alessandra saw the striker neatly set up Bobby Reid on the volley, but his effort rattled the bar.
Even the departure of Plymouth midfielder Dom Blizzard for 10 minutes with a cut eye failed to really change proceedings with Argyle continuing to retain the ball and make Boro run on the heavy pitch. Ironically the midfielder’s eventual return sparked an upturn in chances for Boro as the hosts finally managed to create some meaningful opportunities in the final third approaching the break.
With six minutes remaining a pass by Whelpdale was dummied for Parrett to run onto and shoot just wide. Then within 60 seconds a shot from Whelpdale took a deflection through a crowd of players but Luke McCormick stuck out a hand and superbly parried to safety.
Ben Kennedy had looked lively in parts up top and he was to have the final chance of the half, firing just wide of the far post after cutting in from the right.
Stevenage began the second half strongly and it was the 18-year old who nearly put Boro ahead two minutes in, latching onto a through-ball and controlling on his chest but his lobbed effort cleared McCormick and his bar.
The hosts won a couple of free-kicks in dangerous positions but Plymouth stood firm and instead created a few half chances of their own on the counter. Tareiq Dennis found the side-netting with a driven cross from the left flank and Dembele denying Argyle superbly with a last ditch challenge after Whelpdale had lost possession in his new left-back role, after Martin had replaced Roarie Deacon for the second half.
Twice after the hour mark Charlie Lee tiptoed his way into the box with some neat footwork only to be challenged before a shot came in. Despite these frustrations Graham Westley’s men kept plugging away, looking to create chances where possible.
Plymouth boss John Sheridan threw on Jason Banton for Ryan Brunt and the striker went close to finding the back on two occasions within 10 minutes of his introduction. His first, 17 minutes from time, saw him find space inside the box and send a low effort agonisingly wide of Day’s far post. Three minutes later a shot from 25 yards had Day parrying to safety before the ‘keeper fell on the loose ball.
Back came Boro and when Pett ran at goal, shimmied onto his left and fired in a low effort it looked like finding the back of the net only for McCormick to produce another fine save.
With the Pilgrims winning a couple of late corners it looked like Boro would have to dig deep and hold on for a deserved point, but instead it was Stevenage who made the breakthrough and found the game’s winning goal with 88 minutes on the clock.
Breaking from a Plymouth attack, Parrett found Andrade with a wonderful through-ball and the winger did the rest, outpacing the defence, dribbling towards goal and tucking the ball under the advancing McCormick to send The Lamex into raptures.
Four minutes of added time came and went with only an Alessandra effort over the bar causing a momentary cause for concern. Stevenage held on for their third win in four and now look forward to hosting Newport County at home on Saturday, looking for another win which could see them into the playoffs.
Stevenage: Day, Henry, Dembele, Walls, Deacon (Martin 46, D.Johnson 85)), Pett, Parrett, Walton, Whelpdale (Andrade 76), Lee, Kennedy
Subs not used: Beasant, Bond, R.Johnson, Conlon
Plymouth: McCormick, Nelson, Alessandra, Blizzard, Dennis, McHugh, Reid (Cox 80), O’Connor, Talbot, Brunt (Banton 64), Hartley
Subs not used: Bittner, Mellor, Purrington, Norburn, Harvey
Referee: Andy Haines
Attendance: 3,022 (630 away)