IT happened in the cruellest of manners but despite dominating the game for long periods, Argyle slumped to defeat at Glanford Park.
It all started so well with Jamie Mackie giving the Pilgrims the lead after 25 minutes from Yala Bolasie's crisp cross.
Scunthorpe's Garry Thompson drew his team level soon after though and Martyn Woolford's second-half effort, wildly deflected away from it's intended destination, evaded the fantastic Stockdale to give the home side the points.
On paper, it does not look good as the Pilgrims now find themselves again seven points from safety.
Paul Mariner made just one change from the thrilling Westcountry derby on Tuesday night with Burnley loanee Richard Ecksersly preferred to Chris Barker.
Scunthorpe, beaten at leaders Newcastle on Wednesday made a flurry of changes for the visit of the Pilgrims.
Sam Togwell and Matt Sparrow came into the midfield for Michael O'Connor and Josh Wright, in defence, Rob Jones was dropped for David Mirfin and up top, Martyn Woolford stepped in for Jonathon Forte.
This was a must win game for both sides and the blanket of rain descending onto the already boggy pitch before kick-off made for conditions stereotypical of a relegation six-pointer.
The game sprung into life early on when Argyle, kicking towards the Green Army and with the wind in their favour, had a wonderful chance to open the scoring after just 65 seconds.
Árnason's long throw found Réda Johnson in the middle and his flick-on fell to Mackie who's shot from inside the six-yard box was straight at Iron 'keeper Murphy.
But after that near elation came near heartbreak when Scunthorpe's Paul Hayes broke into the area with men to spare. Damien Johnson made a crucial tackle which only found Woolford on the edge of the area.
The winger twisted and turned and got a shot off which was fantastically saved, low to his right, by Stockdale.
The man on loan from Fulham has received his fair amount of criticism in the last week but that save showed exactly why Paul Mariner has kept his faith in the 24 year old.
After that let off, Argyle began to really dominate and a flurry of corners followed. Réda Johnson was the man really causing trouble in the Scunthorpe defence though, his height and strength in the air proving a constant thorn in the side of the home side.
An injury for Eckersley after 20 minutes forced the introduction of Chris Barker to make the team into the same side that beat Bristol City on Tuesday night.
The refereeing had been pernickety but the decision not to let Árnason back onto the pitch after receiving treatment as the Pilgrims prepared to defend a corner was bizarre. Mariner was just as unimpressed as Árnason himself but that little misdemeanour was soon forgotten when the Pilgrims took the lead.
Having received the ball on the halfway line, Bolasie turned and made for the by-line. He beat his marker with electric pace and despite being under pressure released a low, driven cross into the area which found Mackie for the simplest of finishes from the middle of the six yard box.
It was no more than the Pilgrims deserved for their effort and work rate but it was clear that one would not be enough and just a few minutes later, the Iron were level through Thompson who just beat Fletcher to quick Sparrow free-kick.
It was time for the Pilgrims to show their character and almost straight from kick-off Mackie had a shot that sailed just over.
But with their tails in the air, it was the Iron who were now beginning to contribute to the match, playing some nice football in the final third before Woolford forced Stockdale into action one-on-one despite being a good two yards offside when he received the ball.
The injustice of that little incident seemed to spur Argyle on and just a minute later, Bolasie's pace again caused problems and eventually a shot from the youngster forced Murphy into a save.
His midfield play wasn't too shabby either and on the verge of half-time his superb little dink found Wright-Phillips in the channel. The striker cut in, looked to have found space, but put his shot straight into the hands of Joe Murphy.
The second-half began in mirror image of the first with Scunthorpe enjoying a decent spell of pressure. Woolford had a good chance to score at the far post but miss-kicked for a goal kick.
Thompson was the next Iron to threaten when he beat the offside trap to go clear on goal. Had he taken the shot on his right foot, he'd have probably of scored but instead, he cut inside, allowing Barker time to recover and clear.
Just minutes later, Wright-Phillips had another glorious chance to double his Argyle account. Capitalising on a defensive mix up, BWP was in like a bullet but fired his shot just over.
Argyle were beginning to how what they could do on the attack when Bolasie beat Wright on the left-hand side. Unable to deal with his trickery, Wright was forced into the foul which Damien Johnson jogged across to take.
Johnson's free-kick was deep but found Clark who great volley was saved well by Murphy. But against the run of play came the sound of Green hearts breaking.
Latching onto a through ball, Woolford controlled well, cut inside and saw his shot take a wicked deflection off Barker and into the far corner.
It was a cruel, cruel way to concede with the original course of the ball easily covered by Stockdale.
But they weren't beaten yet as Wright's inability to handle Bolasie resulted in yet another free-kick from the left. Yala himself whipped it in but Árnason's header went just wide.
Despite the pressure, Argyle still had to be awake as Stockdale made another great save from Paul Hayes on the counter.
Rory Fallon made his entrance with 15 minutes to go but juut minutes after, Árnason picked up a knock and made signals to the bench to come off.
With all three substitutes used, the big centre-half was forced to continue, albeit in the midfield as Argyle opted for 3-5-2.
Then came the chance of the game when an Árnason long throw wasn't dealt with by the Iron defence. It bobbled around until it fell to Bolasie, who with the goal at his mercy, somehow miscued and allowed Wright to clear.
That, unfortunately, was to be it as 4,500 United fans rejoiced at the final whistle.
Scunthorpe (4-4-2): 1 Joe Murphy; 2 Andrew Wright, 15 David Mirfin, 32 Michael Raynes, 3 Marcus Williams; 7 Matt Sparrow, 4 Sam Togwell (capt), 8 Grant McCann, 16 Martyn Woolford; 11 Garry Thompson, 9 Paul Hayes (14 Jonathan Forte 85). Substitutes (not used): 5 Kenny Milne, 12 Rob Jones, , 18 Michael O'Connor, 20 Donal McDermott, 22 Josh Lillis (gk), 24 Matt Godden.
Booked: Woolford 38,Hayes 73, McCann 86.
Argyle (4-4-2): 21 David Stockdale; 2 Karl Duguid, 11 Kári Árnason, 22 Réda Johnson, 23 Richard Eckersley (15 Chris Barker 21); 6 Chris Clark (32 Joe Mason 66), 4 Carl Fletcher (capt), 7 Damien Johnson, 36 Yala Bolasie; 8 Jamie Mackie, 10 Bradley Wright-Phillips (14 Rory Fallon 76). Substitutes (not used): 1 Romain Larrieu (gk), , 20 Luke Summerfield, 25 Alan Judge, 29 Bondz N'Gala.
Booked: R Johnson 48, Mackie 85.
Referee: Anthony Taylor (Cheshire).
Attendance: 5,153 (426 away).