Fiore entered stage left with less than half an hour to go and the Rebels trailing to a 49th minute goal by Harriers full back Marcus Bignot.
On for Danny Bolt whose service in both open play and at comers had let the Rebels down, Fiore's first touch set the scene. In came a whipped cross from the flank, trapped by Gary Abbott with his back to goal. And in charged Mark West to hammer a shot from the edge of the area which a diving Fred Barber just managed to palm away.
The Fiore-West combination struck the crucial blow in the 66th minute from a free kick outside the box on the right. A dummy run from Gary Blackford was followed by a delicate inswinger for West to flick home just inside the far post after ghosting in to a free position just inside the area.
Harriers, who had the Rebels on the back foot for long periods of the game, should really have regained the lead when Paul Davies, the Conference's all-time biggest goalscorer, was presented with an open target.
He fluffed it and Harriers were made to pay when Fiore delivered to the far post in the 72nd minute for the stooping Blackford to head under Barber's falling body. The victory, coming as it did after Saturday's under-performance at Halifax, was all the better for being hard-won.
Conditions were difficult, with the lush grass playing treacherously thanks to two torrential downpours, one in each half. But Slough's travelling fans beat a relentless tattoo which drowned out even the thundering of the rain on the steel canopies under which they huddled in hope.
The early signs, despite the six-five corner count in the first half, were not good. Harriers enjoyed more freedom in the midfield and had, in Lee Hughes especially, a frightening turn of pace. The Rebels chipped away with their build ups, but moves all too often broke down around the area. The slowness of the ball across the surface saw Slough leave the ball short of the man too frequently on a day when more weight was clearly needed.
Harriers, meanwhile, forged the clearest chances and exposed Paul Wilkerson's weakness when coming for the ball in the air. Despite all this, Slough had perhaps the best chance of the half in the 33rd minute. A comer hit short ended with West backheeling for Bolt to cross for Hercules who was flattened by the windmill of flying arms and legs.
The ball came out to the lurking West who hammered it back, only for it to be desperately hacked off the line by Steve Prindiville. Chances either side of the interval came and went, but Slough looked as though they had completely blown it when Bignot was allowed an unchallenged 40-yard run through the middle.
Bignot fended off a weak attempt to stop him before curling the ball inside the far post. But fresh legs, fresh eyes, fresh opportunities have worked wonders before, and Fiore's introduction fitted the bill. It also showed the fragility of a one-goal lead.
As McDermott had said after leading Rushden & Diamonds by that slender margin six days earlier - whoever scores the second goal will win the game. Victory on Monday proved the adage rings true and showed, moreover, that McDermott's squad had the character not to roll over.
Kidderminster Harriers Line up
Barber, Bignot, Prindiville, Weir, Brindley, Yates, Webb, Willetts, Hughes, Davies, Doherty. SUBS: Olney (74, for Davies), Casey (81, for Willetts), Deakin.