Darren Ferguson's men overturned an interval deficit to plunge rock-bottom Plymouth a step closer to relegation, but only after Lewis had brilliantly kept out substitute Luke Summerfield's spot-kick at the death.
Earlier goals from defender Ryan Bennett and star striker Craig Mackail-Smith - who struck his 30th goal of the season - did the trick for a Posh team chasing an instant return to Championship football.
Visitors Plymouth were terrific in the first period and eventually hit the front seven minutes before the break when skipper Carl Fletcher poked home a low Simon Walton free-kick delivery.
Posh needed just 10 minutes of the second half to level when Bennett was picked out by an exquisite left-wing cross from Grant McCann and powerfully headed home and Mackail-Smith steered a low Mark Little cut-back under the body of goalkeeper David Button for the home side's second on 70 minutes.
Posh needed a stunning Lewis save to keep out a Walton header soon after and then relied on their big number one again at the death to save Summerfield's penalty after Miller had pointed to the spot when Stephane Zubar went down in the box.