Their witless chanting echoed around the stands in a manner that reflected poorly on Kent and their stewarding operation, such as it was.
It is not known what prompted the abuse, or why Kent did nothing to eject these people.
Plunkett simply responded in the best way possible.
For a second year running, he produced a match-winning performance at the quarter-final stage, earning Yorkshire an 11-run win and a semi-final against Surrey at Headingley on August 28.
Last year, it was Essex who felt the force of Plunkett’s power in the quarter-finals, the all-rounder scoring an unbeaten 49 from 32 balls to lead them to an unlikely par total at Chelmsford.
He followed that up with his side’s best figures of 3-58 as Yorkshire won by 20 runs.
Last night, after Yorkshire scored 256-9 from their 50 overs, Adam Lyth top-scoring with 88 and England vice-captain Joe Root contributing 45, Plunkett returned 4-52 from 10 overs in another decisive intervention.
He began with the wicket of Sam Northeast, caught at mid-wicket to end a dangerous stand of 50 with Joe Denly for the second wicket after David Willey trapped Daniel Bell-Drummond in the second over of Kent’s reply.
After Steve Patterson got Sam Billings lbw, Plunkett followed up by removing Denly, who skied to Willey in the mid-wicket region.
Darren Stevens, playing his 500th match for Kent, and Alex Blake added 86 in a fifth-wicket stand that looked to be putting the hosts on course for victory before Willey had Blake caught behind for 50.
But the key moment of the night came with the total on 180-5 in the 37th over when Plunkett produced a remarkable catch, diving one-handed to his left, to remove Stevens for 54 off his own bowling.
Plunkett took a simpler caught-and-bowled to dismiss Will Gidman before Adil Rashid and Willey closed out the match.
Victory kept Yorkshire on course for an historic treble of County Championship, Royal London Cup and NatWest T20 Blast and delighted their one-day captain, Alex Lees.
“We knew this was going to be a real tough game,” he said.
“Kent had a home quarter-final for a reason, and I thought Adam and Joe played brilliantly with the bat. We battled really hard to get a good score, and our bowling was exceptional. The match-winning performance was Liam’s. He was brilliant.”
Before a near-capacity crowd of 6,000, who watched in glorious weather, Lyth got Yorkshire off to a flier after the visitors lost the toss.
Lyth flicked former Yorkshire pace bowler Mitch Claydon to the mid-wicket boundary and square-drove his next ball to the backward-point rope.
He also lofted Claydon for six over long-off and Lees hammered him for a straight boundary before the captain was first out to the final ball of the fifth over.
Claydon dropped short and Lees pulled down the throat of Blake on the mid-wicket rope.
Lyth and Root ticked along in seemingly effortless manner, scoring at just under five an over without the hint of risk.
Root worked the ball around unfussily and it came as a surprise when he was second out with the score on 117 in the 24th over, the victim of another catch at deep mid-wicket by Blake, this time off Charlie Hartley.
Hartley, a 22-year-old right-arm pace bowler, was playing only his fourth List A game. He completed a notable double when he followed the wicket of Root with that of Bairstow, who went for nine when he chipped to Bell-Drummond at mid-on.
Lyth looked set for his third century in the competition but flirted at one outside off stump from Coles and was caught behind.
Yorkshire lost two wickets with the score on 191 in the 39th over, bowled by Will Gidman, when Tim Bresnan picked out cover and Willey was strangled for a golden duck.
Plunkett sliced Coles to cover, Gary Ballance was run-out for 37 after a mix-up with Rashid when considering a third run, and Azeem Rafiq was bowled by Coles, who ended with 3-39.
Rashid hit a six and a four in the last over, bowled by Claydon, as Yorkshire achieved an advance of four on the total of 252-9 they made in last year’s quarter-final.
Even after Stevens’s departure, Yorkshire could never feel that they had the match won.
Coles swung productively before being smartly stumped by Bairstow off Rashid, who took the ninth wicket to have Hartley lbw.
Willey ended things by pinning James Tredwell, much to the frustration of the individuals beneath the lime tree.