Marcus Maddison created one goal and scored the other to inspire Peterborough to derby glory as they saw off MK Dons 2-0.
Grant McCann's men struck twice in the space of five minutes early in the second half at a rain-lashed ABAX Stadium.
Maddison delivered the corner from which Ryan Tafazolli headed the opener two minutes after the break, and then doubled the advantage himself with a classy individual effort five minutes later as he danced into the box before guiding a low shot past goalkeeper Lee Nicholls.
Dons chief Robbie Neilson responded with a triple change that saw Osman Sow, Robbie Muirhead and Aidan Nesbitt introduced, but it was a member of his starting line-up who had the visitors' best efforts.
Alex Gilbey saw a deflected drive dip just over the bar, had a shot on the turn well blocked by defender Steven Taylor and then tested Posh goalkeeper Jonathan Bond with a fierce low blast.
But as Dons drew a blank, only a fine stop from Nicholls prevented substitute Leo Da Silva Lopes from grabbing a third for the hosts late on.