Plymouth Albion were left to rue a series of missed kicks as their poor record in Cornwall continued.
Albion deservedly led 7-6 at the break thanks to a Herbie Stupple try.
Matt Evans went over two minutes into the second half for the hosts before Bevon Armitage and Sam Hocking struck back to lead 17-11.
But Evans went over again, his fifth try of the season, before Pirates won it with a last-minute penalty from Rob Cook, who returned on loan last week.
Defeat was a big blow for re-shaped after last weekend's campaign-opening 27-22 win over Doncaster.
"As a team, we're trying to bring a new brand of rugby to the Brickfields," said winger Rhodri McAtee, Albion's summer signing from Pirates, who was back at the Mennaye Field to face his old club for the first time.
"We have half-backs that like to play an expansive game and, in my opinion, our back row is phenomenal.
"I think a top eight finish is a realistic goal.
"Anything higher, which is also achievable, would be a bonus," he told BBC South West.
Albion head coach Nat Saumi was also a crowd favourite at the Mennaye Field, having spent five seasons with the Pirates.
Cornish Pirates: Cook; Bedford, Riley, Fatialofa, Evans; Hallett, Cattle; Andrew, Elloway, Prescott, Lyons, Johnson, Marriott, Burgess, Maidment.
Replacements: Storer, Semmens, McGlone, Buckle, Kessel, Penberthy, Cheesman.
Plymouth Albion: Messer; McAtee, Bailey, Armitage, Arnott; Roberts, Rowley; Andrew, Vickers, Morton, Beukeboon, Stephen, Stupple, Hocking, Carpenter.
Replacements: Bowen, Cushion, Hopkins, Nasiga, Fisilau, Cowan-Dickie, Ashcroft-Leigh.
Referee: Matt Carley (SRFU).