Steve McLean, on loan from Scottish giants Rangers, could have bagged himself a hat-trick if not more.
United opened brightly, with McLean twice going close to opening the scoring, before United's weakness with defending set-pieces allowed the home side to take the lead.
Neil Redfearn lifted a free-kick in from the right wing, Lee Beevers rose highest at the far post and his header beat Tommy evans.
The Iron responded positively, with McLean going straight down the other end and forcing Bastock into another save.
The breakthrough came just before the break, when former Brigg Town and Hull City defender clumsily felled McLean in the area.
Left-back Kevin Sharp stood up to convert the spot-kick and level the match.
Scunthorpe continued to push in the second half, forcing corner after corner and repeatedly hitting the woodwork.
A 70th minute corner from Sharp saw Torpey crash a header off the cross bar, prompting a massive scramble which was eventually cleared by the Pilgrims defence.
Torpey again hit the woodwork, this time the post, while a late goal from Lee Featherstone was dubiously ruled out for offside after another goalmouth scramble.