Diarra scored his first goal for Portsmouth after his Arsenal move
Goals from Lassana Diarra and Niko Kranjcar put Portsmouth into the FA Cup last 16 at the expense of Plymouth.
Chris Clark had put Argyle ahead when he drove in David Norris's deflected pass, but Portsmouth answered as Diarra drilled in Pedro Mendes's corner.
Kranjcar made it two, hitting home Glen Johnson's pass from eight yards out.
David James saved terrific shots from Norris and Jermaine Easter as Argyle pushed on, but Portsmouth finished strongly to wrap up the win.
Plymouth had got off to a brilliant start against their south coast rivals, profiting from their first foray into the Portsmouth half.
The highly-rated Norris's attempted pass took a huge deflection off Hermann Hreidarsson but found the feet of Clark, who drove the ball beyond James to score his debut goal for Argyle.
There was no let-up for James. Rory Fallon nearly got the better of him just minutes after Clark's opener, but the former England keeper pushed the ball wide.
James saved Easter's shot from the resulting corner and got down quickly to deny Peter Halmosi.
Portsmouth plugged away on the counter-attack but seemed destined to waste their best chances before finally making the breakthrough after 34 minutes.
Pompey's £5m signing Diarra lashed home Mendes's pass, although it seemed to clip Mathias Kouo-Doumbe's right ankle on the way in.
Portsmouth soon broke away to score their second, with Diarra leading the surge up towards the Plymouth goal.
Johnson lifted a pass to Kranjcar, who scooped the ball into the net while Plymouth defender Paul Connolly dithered.
Clark (left) celebrates scoring on his debut for Plymouth
In true cup-tie fashion, Plymouth rallied, peppering Portsmouth's goal once again.
As the Pompey defence stood static, the ball fell nicely to Norris, who took it on his chest and conjured a right-footed shot which was again saved by James.
Halmosi went close for Argyle and James blasted his defenders as he was forced into an acrobatic leaping save to keep out Easter's strike.
Portsmouth got the keeper's message and slowly began to pen Plymouth back into their own half but a third goal did not look likely.
Last week's hat-trick hero Benjani came close when he picked up Kranjcar's pass but then sliced the ball wide.
Plymouth were left rueing their luck in the dying stages and were unfortunate not to be awarded a penalty when the ball bounced up and hit Sylvain Distin on the arm, taking it behind for a corner.
And then Halmosi's hard shot from the left skittered across the face of the goal as Portsmouth stood firm.
Plymouth boss Paul Sturrock:
"I'm delighted for the players. They showed a bit of quality but the final finish let us down.
"We lost to two zany goals; a deflection and a melee in the box, and lady luck did not shine on us at all.
"I'm satisfied with our performance and proud of the team. It was a team effort and every one of them played well. They've not let themselves down."
Portsmouth assistant manager Tony Adams:
"We started sloppily and conceded early again. The goalkeeper was in the game too much for me.
"The Plymouth coaching staff thought it was the best the team have played; they raised their game and we were rattled.
"But the way we came back with fighting spirit was fantastic and that's credit to the players and keeper David James."
Portsmouth: James, Johnson, Pamarot, Distin, Hreidarsson, Lauren (Davis 67), Pedro Mendes (Hughes 90), Diarra, Kranjcar, Nugent (Mvuemba 46), Mwaruwari.
Subs Not Used: Begovic, Aubey
Goals: Diarra 34, Kranjcar 45.
Plymouth: McCormick, Connolly, Timar, Doumbe, Sawyer, Clark (Folly 82), Nalis, Norris, Halmosi, Fallon, Easter (Jutkiewicz 75).
Subs Not Used: Larrieu, Hodges, Summerfield.
Booked: Halmosi, Doumbe.
Goals: Clark 5.
Ref: Andre Marriner (W Midlands)
BBC Sport Player Rater man of the match: Plymouth's Chris Clark 7.53 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.