The reverse fixture in October saw the away side brush aside the challenge with a rewarding 5-1 victory.
The away side scored an early opener through Chepstow striker Harry Thorne, latching on to a fantastic through ball by Ethan Dunscombe. Mardy responded within minutes of the opener from close range, to set up a fantastic encounter.
Chepstow striker and goal provider Dunscombe (pictured) finally got his name on the score sheet with a great long range strike that was fumbled by the home keeper.
With a 2-1 lead, the Chepstow Town defence was again caught cold with a quick Mardy breakaway goal from the halfway line.
The Chepstow Town lead was restored, 10 minutes from the half time whistle, with a fine free kick from the edge of the area by Ben Thomas.
But the lead was short-lived as a Mardy equaliser was put into the back of the net with only minutes remaining on the clock of the first half.
Chepstow Town captain Harry Martyn put the away side ahead minutes into the second half, following a fine corner being delivered in from the left by Dunscombe.
Mardy were awarded a very dubious penalty after the restart with the referee judging that the Mardy forward had been tripped inside the box.
The resulting spot kick found a fourth equaliser for the home side to set up a tense finale.
Chepstow Town dominated the possession play but a defensive mistake let in the Mardy forward to calmly finish a one-on-one with the Chepstow keeper, making it 5-4 to the home side.
A spirited Chepstow Town display saw the away side fight for a late equaliser but they were cruelly denied an injury time leveller by the woodwork following a strike from inside the area by Thomas.