The U’s had an early sight of goal when Zoe Barratt broke through, but saw her shot take a deflection.
United then found themselves a goal down after nine minutes, when Taya Pomfret headed home from a corner, and then struggled to break down a tough Argyle side before the half-time break.
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Five minutes into the second period though, Barratt found Leah Burridge, who fired in from long range to beat Argyle goalkeeper Lily Felgate.
Phoebie Poole shot wide just after the hour mark, and Barratt then gave the U’s the lead midway through the second half, tapping home from a Poole cross after United won the ball back on the edge of the Argyle area.
Vita Budaieva, on as a substitute for Poole just five minutes earlier, secured the three points with five minutes to go, when she won possession just outside the Argyle penalty box, and then rounded Felgate.
U’s manager Sam Rose said: “It was loads better in the second half, and I think that the changes that we made really helped, not just bringing players off the bench, but in terms of our structure, just to open up the play a little bit.
“Plymouth were 1-0 up and sat deep behind the ball, and made it really difficult for us, so that was our challenge having conceded earlier in a set play.
“We’re over the moon that we managed to score three goals in the second half.
“Our skipper Leah got us back into the game with an effort from way out, which gave us the life we needed to have a bit of belief about us, to go and try win the game.
“Vita’s come off the bench to ultimately put the game to bed, which is great.
“It’s important if you don’t start, you’re ready to come on, and everybody did that.
“We’ve got Jenna Legg to come back in, which is going to be a massive boost for us, and we might have another player through the door between now and the weekend.
“We’re happy with where we’re at, but to add Jenna plus another into the squad is really going to help, and that’s a positive.
“It was a good win, and hopefully we can build on that, and keep ticking on.”
United lost their opening game of the Southern Premier Division season 3-0 against Ipswich Town, and are next in action on Wednesday night at Abingdon United in the National League Cup determining round.