Jed Wallace secured a draw for Portsmouth after Yassin Moutaouakil's mistake gifted Plymouth the lead in a tightly contested game at Home Park.
The hosts had started brighter but Pompey eventually played their way into the game with John Marquis missing a good chance when put through by Ricky Holmes.
But Plymouth took the lead when Moutaouakil attempted to chest Durrell Berry's cross back to the goalkeeper but got it horribly wrong and gave the ball straight to Conor Hourihane who beat Trevor Carson at his near post for the opener.
Pompey had numerous chances to equalise after that with Sonny Bradley having his free header blocked and Marquis forcing an outstanding save from Luke McCormick with a fierce drive that was heading for the top corner.
The score was eventually levelled on 39 minutes when Jed Wallace was given acres of space to receive the ball from Marquis, adjust his feet and produce a cool finish with no Argyle players around to stop him.
Pompey had been the better side when Plymouth took the lead and might have been expected to put Argyle to the sword in the second half but that never happened.
The half was drab until the final 20 minutes when Marquis headed Agyemang's cross onto the roof of the net. Plymouth had a couple of chances late on with Moutaouakil producing a match-saving challenge to stop Ruben Reid tapping in Lewis Allesandra's cross and then Carson blocked Tyler Harvey's goalbound shot with his legs late on.
That was the last meaningful chance of a match that should have been there for the taking for Pompey after the equaliser and it's disappointing that the initiative wasn't taken and Guy was happy to seemingly sit back and let things happen whilst John Sheridan made two changes that swung the game in his side's favour.
As soon as Lewis Allesandra came on the midfield battle turned for Plymouth as Johnny Ertl was struggling to keep pace with him which allowed him to ping passes out to the wide men who were causing Pompey problems all game.
My personal opinion is that if Pompey are going to make a challenge for promotion this season then the mentality of this club has to change. To be happy with a draw against a side that had taken one point from twelve and were there for the taking is almost settling for mediocrity and despite everything that has happened in the last few years, this club should be aiming higher.
Pompey Player Ratings
Trevor Carson: 6 - One good save late on even if it was fortunate.
Yassin Moutaouakil: 6 - Error and crucial challenge sums him up perfectly. Inconsistent.
Sonny Bradley: 6 - Might have scored on another day.
Bondz N'Gala: 6 - Solid if unspectacular.
Joe Devera: 6 - Did a job but not his natural position.
Jed Wallace: 7 - Nicely taken goal.
Johnny Ertl: 6 - Lost out later on when the Plymouth substitute came on.
Therry Racon: 7 - Won the ball back often.
Ricky Holmes: 7 - Can deliver a good cross.
Patrick Agyemang: 7 - Battering ram but passing was off.
John Marquis: 6 - Worked hard but finishing let him down.