The Martyrs arrived at Holloways Park missing key players in Nathan Davies, Ryan Newman, Steve Williams and Matthew Harris, but were able to give a debut to former Southampton forward Kayne McLaggon – and he was to have a big say in his side getting back into the match in the second half.
It was Slough who had the best chances of the first half, with Smith denied by goalkeeper Craig Morris after a poor clearance had fallen straight to him. Ollie Burgess drove a shot in from 25 yards, but Morris saved comfortably.
Somerville made a smart save down low to his left to hold a McLaggon shot, but Slough’s best chance came from the head of Scott Harris, but he sent Darren Wheeler’s corner over the bar.
Slough edged ahead four minutes before half time through a superb Burgess free kick. From a central position 20 yards out, Burgess picked out the top corner, leaving Morris with no chance.
The second half began with the visitors replacing Zaki Misbali with substitute Ian Traylor. After Smith had turned with his back to goal and fired a couple of feet wide, Kris Leek shot wide from 25 yards after Burgess lost possession.
The dangerous McLaggon then began to make his mark on the second half, first embarking on a solo run down the left which ended with him bringing a save out of Somerville, and then winning a penalty shortly after, with Burgess sliding in but bringing him down. McLaggon himself stepped up to put his kick down the middle of the goal – Somerville almost got a leg to it, but was left frustrated.
McLaggon should have put his side ahead minutes later after a mistake from Ryan Lake gave him possession, and he burst clean through on goal, only to be denied by Somerville, who spread himself well to block.
David Woozley became Slough’s fourth player to be booked in the match, and tempers threatened to boil over on both sides. In truth, referee Michael Taylor had contributed by frustrating both sides with some of the yellow cards shown. In the first half, Ben Edwards, Jamie McClurg and Ollie Burgess were all booked, with several Merthyr players also on the receiving end in a match which was keenly contested but hardly ill-tempered.
The late drama began with the award of a penalty to Slough with around ten minutes remaining. Woozley was judged to have been fouled by Jamie Rewbury in the box, with the defender climbing on top of the Slough skipper. Taylor awarded the spot kick, and dismissed Rewbury for a second bookable offence. Worse was to follow within seconds, as Paul Keddle was also dismissed after being shown a second yellow card for dissent, before the penalty was even taken.
When Smith eventually stepped up, he was far too casual in his approach, and his weak effort was easily saved by the legs of Morris. McClurg managed to get to the rebound, but sent his shot wide.
Had Smith converted the penalty, the task would surely have been beyond the nine men of Methyr, who were battling to stay in the tie and hold on to a replay, but unbelievably the Welsh side took the lead with five minutes remaining.
With Slough committing men forwards in search of a goal, the visitors caught the hosts on the break, and substitute Traylor was allowed to run through the defence and round Somerville before slotting in from a tight angle to spark wild celebrations and send his side through in unlikely circumstances.
Merthyr Town Line up
Craig Morris, Dan Summerfield, Alun Griffiths, Jamie Rewbury, Gary Colborne (c), Paul Keddle, Aaron Cornwall, Kris Leek, Ryan Laville, Zaki Misbali, Kayne McLaggon. Subs: Kenny Morgan, Andrew Thomas, Ian Traylor, Ryan Prosser, Giles Cutlan.