Twice Orient fell behind with strikes from Antoni Sarcevic and Matty Kennedy, but the never-say-die spirit of the O’s was in evidence from Danny Webb’s side once again, as Massey scored a sublime first leveller and then smashed in a second with two minutes left.
Five minutes of stoppage time was awarded and Massey was the catalyst as his ball in found Koroma, whose shot was blocked and then fell to Semedo who lashed it home to send 156 Orient fans wild in Argyle.
The O’s made three changes from the side which drew 1-1 at Yeovil Town on Saturday with Yvan Erichot and Nigel Atangana missing out through groin injuries, while Rowan Liburd dropped to the bench with Callum Kennedy, Josh Koroma and Sam Dalby coming in respectively.
Striker Paul McCallum missed out after a recurrence of his knee injury and Sammy Moore remained absent through illness.
Plymouth had the early pressure and a teasing inswinger from Graham Carey was well watched by the O’s defence as Orient avoided any early calamity.
Webb’s side got going with six minutes on the clock and the on their first attack the O’s two youngest players almost combined to good effect.
Myles Judd’s ball in found the head of Dalby, but Pilgrims skipper Luke McCormick was able to gather.
Minutes later, Collins and Massey worked another opportunity with the former seeing a 20-yard shot held by the home goalkeeper as Orient began to work their way into the game nicely.
However that suffered a setback 10 minutes when former O Gary Sawyer delivered a ball into the box which was nodded back across goal by Ryan Taylor and Antoni Sarcevic was there to apply the finishing touch and sweep it past Sam Sargeant.
Graham Carey continued to cause the O’s problems and another wicked inswinging free-kick found the unmarked Yann Songo’o but the midfielder could only glance his header wide of the goal.
Despite the setback Orient remained positive and a fierce left-footed shot from Dalby saw McCormick push the ball behind for a corner.
The visitors continued to play the ball around nicely and a well-worked move ended with the ball at skipper Liam Kelly’s feet.
He took a couple of touches before firing an effort goalwards which was deflected and made things slightly more awkward for McCormick in the Pilgrims goal, though he was able to keep hold of the effort.
Plymouth remained dangerous, though. Matty Kennedy showed Judd a clean pair of heels down the left and his ball in found Taylor however his first time effort was over the bar.
The game edged towards half time, and Massey remained the main outlet for the O’s and we witnessed yet another piece of Gavin Massey magic two minutes before the break.
The winger-turned-forward picked the ball up down the left and after a mazy run he jinked his way inside and curled a sublime shot into the bottom corner to level things up.
Five minutes of stoppage time were awarded and in that time the hosts had a great opportunity to level the scores. A ball in from the left found the head of Songo’o who nodded it back across goal and somehow from two yards out Jakub Sokolik blasted over when it looked easier to score.
On the stroke of half time the ball then fell kindly to Fox outside the area and his right footed effort was inches wide of the target as the teams went in 1-1 at the break.
No changes were made at the break by either side and after a quiet opening few minutes the O’s created the first opening when Koroma did well to get the better of Sawyer, but no-one could get on the end of his pull back.
But with 54 minutes on the clock the hosts were back in front when Kennedy produced a curling finish from just outside the box past the outstretched dive of Sargeant and the O’s had it all to do again.
Orient were undeterred once again and a raking cross-field ball from Collins picked out Kennedy, who in turned played the ball into the box. Dalby met the cross with a header only to see it deflected over the bar.
Kennedy then almost added a third for the hosts and it came in near identical style as he tried his luck from just outside the box only to be denied by the post.
Orient kept going and Massey led a quick breakaway for the visitors. He played in Collins down the right who returned the favour.
The former Colchester man then ran through on goal before seeing his shot blocked behind for a corner.
Koroma worked the next opening after Bulvitis lost possession and the winger saw his effort pushed behind by McCormick, before Dalby flashed over and Koroma curled another strike into the hands of Pilgrims keeper as the game entered the final 10 minutes.
Bulvitis, without even knowing it, almost put the hosts out of sight when a corner was cleared, but it hit the defender and then the far post before Sargeant was able to hold on the line.
The O’s launched a swift counter which put Dalby through on goal. He lost possession but Massey dinked a ball over for Koroma who saw a shot blocked behind for a corner, with some believing it to be handled.
Another set piece followed as Orient looked for a late leveller and they got just that from the man Massey once again as the ball fell to him, just like it did at Yeovil, and he lashed it home once again for his fifth goal in four games.
It wasn’t done there though as four minutes into five minutes of stoppage time the O’s had picked up a massive win.
Massey was again instrumental as his run down the right created an opening for Koroma. His shot was blocked but the ball fell to Semedo and the Portuguese winger lashed it home to spark wild scenes of jubilation in the away end and seal a big three points for the O’s.
PLYMOUTH ARGYLE: McCormick, Threlkeld, Sokolik, Bradley (Bulvitis 15), Sawyer, Songo’o, Sarcevic (Donaldson 38), Fox, Kennedy, Carey (Jervis 80), Taylor.
SUBS: Dorel, Slew, Spencer, Tanner.
GOALS: Sarcevic (10), Kennedy (53).
BOOKED: Kennedy (82)
ORIENT: Sargeant, Judd, Hunt, Parkes, Kennedy, Koroma, Collins, Kelly, Semedo, Massey, Dalby.
SUBS: Grainger, Mezague, Liburd, Moncur, Adeboyejo, Alzate, Clark.
GOALS: Massey (43) (88), Semedo (90+4)
BOOKED: Kennedy (37), Parkes (39), Hunt (41), Semedo (52), Kelly (70)
REFEREE: Christopher Sarginson.
ATTENDANCE: 8,054 (154 Leyton Orient fans)