It is possible to have a philosophical debate about whether Whitehaven are ready for Super League; what no one can say after this is that they do not want it. "You couldn't have written the script for that,'' their coach, Steve McCormack, said. "They started off outstandingly well and we were very edgy, but in the second half everyone stepped up.''
Castleford started the game playing like a Super League team, sweeping Whitehaven away with pace and intensity. Brad Davis was at the heart and it was his perfectly weighted kick into the corner that brought Waine Pryce their first try after just two minutes.
Andy Henderson put through the kick for their second after 10 minutes, a convenient deflection off a defender setting the ball up for Deon Bird. Even when Haven prevented a third try soon after, it was at the cost of Joel Penny being sent to the sin-bin forhanging on in the tackle after getting back to stop Tom Haughey.
Craig Huby put over the penalty for his second goal and Castleford were even more firmly in control when Davis' superb pass sent Michael Eager through a yawning gap.
But from a knock-on by Michael Platt, Whitehaven finally made the breakthrough that gave them some hope at half-time.
Again it was a clever kick, this time from Penny, that did the trick, with Carl Sice following up to touch down. The same combination struck straight after the interval. Nanyn's second goal made it a six-point gap, stretched to eight when the same player was caught offside going for an interception. Whitehaven got the scent of blood when they capitalised on a period of pressure through Craig Chambers charging on to Leroy Joe's short ball.
The scores were level when Joe's pass allowed Craig Calvert to squeeze in at the corner. Now the momentum was all with Haven and, after Bird was sent to the sin-bin for time-wasting, Aaron Lester made up for a handling mistake by sending Nanyn over.
As time ran out, Cas were opened up again down the same flank and Nanyn exchanged passes with Wesley Wilson for his second.
Castleford must beat Halifax to reach the Grand Final and will spend much of this week wondering how they lost. "We played as we trained in the first 20 minutes,'' their coach, Dave Woods, said. "In the second half we played dumb football. I can't understand the transformation.''
Whitehaven: Broadbent, Calvert, Seeds, Nanyn, Wilson, Joe, Penny, Pandy, Site, Fatialofa, Miller, Hill, Lester. Substitutes used: Rudd, Summers, Chambers, Jackson.
Castleford: Platt, Pryce, Eagar, Hepworth, Blanch, Davis, Henderson, A Watene, Smith, Huby, Haughey, Crouch, Bird. Substitutes used: Ain, Vowels, Fletcher, F Watene.
Referee: S Ganson (St Helens).