Matias Moroni threw himself sideways and upside down, and somehow bundled Louis Rees-Zammit into touch.
Argentina were leading 19-17 with time ticking down for Wales to launch a decisive rally, Rees-Zammit flew in for the corner – and Moroni swept him away.
The Pumas raced up the field, kept Wales at bay, and then Nicolas Sanchez pounced on a telegraphed pass from Sam Costelow.
The one-time team-mate of Jonny Wilkinson at Toulon, a veteran of the 2015 and 2019 campaigns, intercepted and raced half the field for a score to flatten Warren Gatland’s Wales.
Emiliano Boffelli slotted the extras, then a fifth penalty at the death, and Argentina stormed into a third appearance in the World Cup’s last four with a famous 29-17 victory in Marseille.
Instead of Gatland leading Wales to a third semi-final in four tournaments, former Australia boss Michael Cheika sent the Pumas through.
Cheika and company will care not one jot for Wales however, having finally delivered a performance worthy of this World Cup, at the fifth time of asking. Argentina will now face either Ireland or New Zealand in a seriously daunting semi-final in Paris.
Referee Jaco Peyper had to be replaced by England's Karl Dickson just 15 minutes in, due to an injury. Wales will be left to rue Dickson's later quizzical call not to impose any sanction on Guido Petti, who cleaned out Nick Tompkins at a ruck, clashing his shoulder into the Saracens centre’s face.
Petti was judged to have been completely mitigated by Tompkins slipping out of a tackle and falling at the last, with the Pumas lock committed to joining the breakdown and unable to change position.
Tompkins had to be sent for a Head Injury Assessment (HIA) however, leaving Wales furious that their centre could suffer a potential head injury and the perpetrator go unpunished.
Argentina pressed on undeterred, and pressed home their momentum. Prop Joel Sclavi powered home from a series of tight drives, after Argentina punted three kickable penalties to the corner.
The 29-year-old refused to content himself with forming part of La Rochelle’s Champions Cup win in 2022, instead seizing more glory in France at a pumping Stade Velodrome.
Wales tried to find an answer, but could not deliver. Moroni thwarted Rees-Zammit and winning confidence suddenly oozed through Argentina.
The Pumas beat Wales back, pushed up the field, then Sanchez picked off that stray Costelow pass to kill the contest.
Wales will feel that it should all have been so different. Gatland’s men raced into a 10-point lead, thanks to Dan Biggar’s try, conversion and penalty.
Biggar sent George North through the line on an inside ball, the outside centre popped off the ground to Gareth Davies and the scrum-half sent his half-back partner in under the posts.
Argentina were overwhelmed, but refused to relent. Boffelli slotted two penalties to send Wales into a 10-6 lead at half-time.
The Pumas returned after the break fired up and ready to make their mark. Wales slept on their foes’ comeback, with Boffelli adding two more penalties to steal a 12-10 lead.
Tomos Williams ghosted past an unposted ruck to score Wales’ second try and wrestle back the lead at 19-17. But then came Moroni to dismiss Rees-Zammit, and Sanchez to pick Costelow’s pocket – and Wales were out.
All hail former London Irish hooker Agustin Creevy, the 38-year-old stepping off the bench to finish the job.
The Brive-bound front-rower is in his fourth and last World Cup – but now has two more weeks playing for the Pumas in France before hanging up his boots.