When it was all over, City were celebrating a second win in seven starts, while Argyle were deeper in trouble at the foot of the Championship.
The goal rush began in the 73rd minute with City's Danny Haynes sliding in a ball from attacking wing-back Jamie McAllister.
Within two minutes Jamie Mackie rallied Argyle with a rasping right-footed drive, before they were then plunged into despair by two strikes in a minute.
McAllister crossed a short corner for fellow defender Jamie McCombe to head City back in front, before Nicky Maynard sealed the win on the edge of the box, his left foot bringing him his eighth goal of the season.
It was hard for Argyle to accept. Anxious to please new coach Paul Mariner, who has returned from America to rejoin the club he first signed for 36 years ago, they were always pressing City without quite finding their shots.
But they forced home manager Gary Johnson to rethink his strategy.
After Saturday's home draw with Peterborough he had purged his midfield - only Cole Skuse survived the cull - and started with three forwards.
But his team struggled with the new formation and after half an hour it was scrapped, Costa Rica striker Alvaro Saborio was withdrawn and Lewis Carey sent on to stiffen the defence.
He had been dropped to the bench to rest a suspect knee, but there was no sign of any problem as he brought stability to the back four.
Until the goals, there had been little to test either keeper. Argyle centre-back Shane Lowry had gone close with a header from a corner.
For City Paul Hartley had so nearly broken the stalemate with a curling free-kick from 25 yards.