McManus 22, Franks 90
UNCERTAINTY in each penalty area cost the Pilgrims dear in their bid to avoid relegation from the Championship.
Back-to-back wins from their Easter games would have given hopes of playing in the second tier come August a huge boost.
However, after conceding a sloppy, soft goal to Stephen McManus midway through the first half, they were unable to get back into game that, if not must-win, was precious close to it.
A penalty award just before the interval seemed to give the Pilgrims a way back in but Alan Judge's attempt was well saved and the chance went begging.
Jonathan Franks rounded off Argyle's misery when Boro broke from a corner that had seen Argyle goalkeeper David Stockdale go up, leaving Franks to slot into an unguarded net.
Argyle had shown just one change to the initial line-up that had started Saturday's 2-1 come-from-behind victory at Doncaster.
The switch was on the left side of midfield, where Joe Mason, scorer of the leveller at the Keepmoat, was handed only a third start of his breakthrough season in place of Yala Bolasie, whose energy levels have taken an unsurprising battering lately.
Middlesbrough had done the Pilgrims a favour at the weekend by not letting Crystal Palace beat them and made five changes from the starting 11 in that 1-1 draw on Teesside.
Defender Andy Taylor and captain Gary O'Neil were injured, while forward pair Scott McDonald and Chris Killen were dropped.
The line was led by the itinerant Leroy Lita, and another of the host of Scottish Premier League players imported by ex-Celtic manager Gordon Strachan since his arrival at the Riverside, Lee Miller.
Kyle Naughton, on loan from Spurs, filled in at full-back, while Julio Arca and Jay O'Shea came in to the engine-room.
The Pilgrims looked nervous or tired, or possibly both, in the opening exchanges and an early raid down the Argyle left was begging for a better finish than O'Shea could manage from Justin Hoyte's low cross and the shot was easily blocked.
What was needed was a chance to open the legs and have a go at the Boro back-line, and that duly came when the persistence of Chris Clark allowed Jamie Mackie to wriggle clear on the right.
Mackie's driven cross was scrambled clear by David Wheater for a corner, from which Réda Johnson powered a header under pressure wide.
The Pilgrims' bright opening dimmed midway through the first half with a goal that could not have been softer, all round. Boro won a free-kick on the right for what appeared a decent Mason tackle on Hoyte, from which McManus was picked out by his Celtic team-mate Barry Robson for a simple header under little challenge.
Argyle responded with a sharpness and a snap in the tackle that they had no right to show, given their tremendous efforts over the last few weeks, but they were unable to break back, Bradley Wright-Phillips' shot into Brad Jones' breadbasket being as close as they came.
That was until Hoyte handled Judge's drive from the edge of the area just before half-time and referee Scott Mathieson awarded a penalty. Judge, who has scored three times from the spot this season, took the responsibility but saw his shot, low to Jones' right, well saved by the Australian.
Arguably the most important 45 minutes at Home Park since the visit of Queens Park Rangers six years earlier saw the Pilgrims try to progress down either flank, but fail to make headway in the centre.
Lita was lucky to remain on the field after an outrageous, and outrageously bad, penalty-area dive, although calling his back- and- up jump actually demeans the term. Anyway, it was the second such piece of cheating by Lita, and he had been booked for the first, less theatrical effort.
Although Argyle huffed and puffed, Middlesbrough never appeared under too much pressure and came close to doubling their winning margin when a swift break ended with Robson pulling out a good save from Stockdale.
BWP was given a break when Paul Mariner threw on Yala Bolasie in an attempt to unlock the Boro defence, moving Mason into the centre alongside Mackie.
It was a Boro substitute, Scott McDonald, on for Lita, who had the chance to put the issue beyond doubt, but the Celtic man (you may be spotting a theme here) failed to get anything on Rhys Williams' cross when the merest of touches from six yards would have seen him score.
Mariner tried Fallon in the van as time ran out on the game and, possibly, Argyle's time in the Championship, but the big Kiwi found the Boro defence as niggardly as his team-mates, which included Réda Johnson playing as a makeshift additional forward.
That tactic, on its own, showed how desperate the Pilgrims were for a point, at least.
It is not over yet, of course, but somewhere, you feel, there is surely an overweight woman warming up her tonsils.
Argyle (4-4-2): 21 David Stockdale; 2 Karl Duguid (capt), 29 Bondz N'Gala, 22 Réda Johnson, 35 Onismor Bhasera; 25 Alan Judge (14 Rory Fallon 74), 7 Damien Johnson, 6 Chris Clark, 32 Joe Mason; 8 Jamie Mackie, 10 Bradley Wright-Phillips (36 Yala Bolasie 62). Substitutes (not used): 1 Romain Larrieu (gk), 15 Chris Barker, 23 Richard Eckersley, 31 Yoann Folly, 33 Kenny Cooper.
Booked: Duguid 75.
Middlesbrough (4-4-2): 1 Brad Jones; 2 Justin Hoyte, 31 David Wheater, 19 Stephen McManus, 33 Kyle Naughton; 15 Jay O'Shea (28 Jonathan Franks 83), 25 Rhys Williams, 17 Barry Robson (capt), 20 Julio Arca; 9 Leroy Lita 7 Scott McDonald 65), 18 Lee Miller (12 Chris Killen 77). Substitutes (not used): 21 Danny Coyne (gk), 23 Jonathan Grounds, 27 Joe Bennett, 37 Bruno Pilatos.
Booked: Lita 41, Hoyte 42, Robson 75, Williams 86.
Referee: Scott Mathieson (Cheshire).
Attendance: 11,770 (674 away).