Three goals down at half-time to a Watford side who'd suddenly discovered a long-missing clinical streak, Tranmere thoughts must have already turned back to their survival fight in League One.
It may have a Watford team that showed nine changes from the one fighting their own relegation scrap in the Premier League - an assortment of fringe first-teamers and youngsters - but the 46-place gap between the two teams in the pyramid could not have been more glaring.
Goals from Tom Dele-Bashiru, Nathaniel Chalobah and Roberto Pereyra were turning this into an absolute cruise for Nigel Pearson's seemingly resurgent team.
But after the break - and you may call it the magic of the FA Cup or whatever you choose - Watford capitulated from their position of absolute strength, beaten down by Tranmere's sheer force of will.
A quite incredible comeback and goals by Connor Jennings, Manny Monthe and Paul Mullin earned Tranmere one of the most unlikely replays you'll ever see in this competition.
What incredible spirit and resolve against mighty odds, with Watford's sheer exasperation boiling over in the closing minutes when Pereyra was sent off for kicking out at Kieron Morris.
It just goes to show you that hope should never be completely abandoned in football.
The Watford first-teamers were wrapped up in cotton wool for next Sunday's six-pointer against Bournemouth as Pearson handed out two debuts and a first start for Dele-Bashiru.
And how the 20-year-old announced himself just 12 minutes in. A product of the Manchester City academy, Dele-Bashiru signed on a six-year contract back in the summer.
He's had to patiently await an opportunity, having played just 18 minutes off the bench in a Carabao Cup tie against Coventry City back in August but showed his pedigree here.
Tranmere's defenders just failed to spot him lurking on the edge of their box following a short corner routine and Dele-Bashiru had no other thought but to shoot when Chalobah rolled the ball to him 25 yards from goal.
As the visiting defenders reacted too late, Dele-Bashiru arrowed a dipping strike into the bottom corner and the crowd responded warmly to the youngster's big moment.
Within another couple of minutes, the contest was seemingly settled. This time Tranmere were the architects of their own downfall, showing really why they're third bottom in League One.
A hopeful ball down the left touchline caused issues for Sid Nelson, who was hemmed in and squared into his own box to debutant Peter Clarke. He inexplicably allowed the ball to run across his body and then played Monthe into trouble.
Isaac Success applied enough pressure to unsettle Monthe, whose prodded clearance fell straight to Chalobah on the edge of the area. His strike was firm and low, picking out the far corner.
'Two-nil on your big day out,' mocked the chipper home fans to the 2,700 travelling fans from the Wirral. Chalobah made a beeline for Pearson on the sidelines and received a hug.
With so many fringe players given an opportunity to catch Pearson's eye, Watford continued to press for more goals and their third arrived just after the half-hour.
Success had by this point been moved into midfield and a neat body swerve was followed by a pass into Andre Gray. The striker turned but couldn't quite twist his body to shoot.
No matter. In came Pereyra to sweep home his third goal of the season and that really should have been that.
18-year-old Joao Pedro received a warm welcome when brought on for his debut at half-time, little wonder given the long wait to reach this point.
The striker signed back in October 2018 but work permit wranglings meant Pedro is only just eligible to pull on the yellow and black shirt.
He was desperate to make an immediate impact, shooting just wide in the opening minutes of the second half after more good work by Success.
Tranmere had only tested Watford keeper Daniel Bachmann with a peppering of long-range shots but gave themselves a glimmer of hope midway through the second-half.
A weak air-kick clearance by Success allowed Corey Blackett-Taylor the chance to redeliver a cross and up popped Jennings to nod home from close range.
Initial celebrations were cut short by a linesman's flag but VAR overturned that decision and allowed the goal to stand.
Tranmere sniffed a route back and Jennings forced Bachmann into a reaction save following an acrobatic flick just a couple of minutes after the goal.
But things got really interesting 12 minutes from time. Tranmere won a free-kick wide on the left that keeper Bachmann failed to collect. Morgan Ferrier helped it back across and Monthe spun to lash the ball home.
Yet, incredibly, there was more. With four minutes to play, Watford substitute Mason Barrett fouled Taylor inside the box. Referee Graham Scott pointed for a corner, but VAR deliberations decided a penalty.
Mullin rammed home the spot-kick gloriously, sparking scenes of indescribable joy among those visitors who'd abandoned all hope at half-time yet saw their team live to fight another day