Even Roberts, though, will concede that his second goal, which effectively won the game, was an outrageous fluke.
It came after Nathan Ellington did well to work an opening on the left. He appeared to have wasted all the good work, though, with a spectacularly mishit shot that was heading well wide.
However, the powerful strike hit Roberts as he tried to evade it and the ball looped gently past Argyle goalkeeper Romain Larrieu.
Earlier, Roberts had given the Championship table-toppers a deserved lead just after the interval, when strike-partner Ellington flicked on goalkeeper John Filan's kick-out. Roberts held off two Argyle challenges before slotting home.
A late diving header by Scottish international Stevie Crawford, after Mickey Evans had won David Friio's cross, gave the Pilgrims some hope, but there was to be no repeat of the previous home game's heroics, when they scored twice in the last two minutes to win.
Roberts would have had a hat-trick had it not been for an unbelievable first-half miss.
Ellington created the opportunity with a cross that looped up off David Worrell's attempted block taking the ball over Larrieu to Roberts.
Despite being unmarked, within one yard of the target in, and front of an open goal, Roberts contrived to head the ball against the post.
Argyle's best first-half opportunity fell to David Norris in the opening minutes.
Worrell's chipped pass to Crawford down the left flank sent the Scottish international away, and, when Filan saved his low shot from an acute angle, Norris could only blast the rebound over the crossbar.