Monchengladbach could have sealed the top spot with a victory but three points now separate the four sides heading into next week’s final round of fixtures.
Lukaku scored twice in nine minutes to make it 3-1, before Plea’s second.
There was late drama as Ashley Young missed an excellent chance to put the result beyond doubt, before a VAR review with seven minutes remaining denied Plea his hat-trick and ensured Antonio Conte’s side remain in this season’s competition for at least one more game.
It had been a miserable Champions League campaign for Antonio Conte‘s winless Inter, currently second in the Italian top flight, with just two points to show from five matches.
Darmian ensured Inter’s early dominance was rewarded as he slotted in from a tight angle after 17 minutes, but Monchengladbach responded well to equalise before the interval.
The momentum shifted again as Lukaku fired in for Conte’s side, two minutes after Lautaro Martinez had struck the post from the edge of the penalty area.
Young’s miss from a yard out looked to have cost Inter, but a VAR review found Breel Embolo in an offside position after Plea thought he had sent the Bundesliga side through as group winners.
Inter had to win to stand any chance of qualification, but they now have every chance of reaching the knockout stages for the first time since 2012.