Wearing the colours of the Italian sponsor, Carrera, in which he won the Tours of France and Italy and a world title in 1987, Roche raced clear in the last 25 kilometres, and that was the last the pack saw of him.
He vanished around a bend and chased down the remnants of an early breakaway, before the mist, hanging over the spa of La Bourboule, shrouded his progress to the top of the Charlannes climb.
He flitted from patch to patch, gaining a minute which dropped to 46 seconds as Viatcheslav Ekimov burst out of the gloom for second place at the summit of the Tour's final hill of note.
Roche had assured everyone that his form had again reached its 1987 peak. 'I just need a victory to prove it,' said the Dubliner, whose 1987 feats had been achieved previously only by the legendary Belgian, Eddy Merckx.
'My attack was a long shot, but there is no other stage finish between here and Paris that suits me. I had tried for a win on Monday. It did not work, so I tried again.'
After his first-week showing that eventually lifted him to third overall Roche lost it all to the Alps. In 400km of mountain passes he had forfeited more than 22 minutes by the evening the Tour reached the Alpe d'Huez summit.
'I dread Alpe d'Huez. It has never been kind to me,' the Irishman said. 'I lost one and a half minutes there to Pedro Delgado when I had the yellow jersey on my back for inspiration.'
Roche has learned to suffer. From the headiness of 1987 he languished either in surgeries or court rooms, awaiting verdicts on a persistent knee injury or his dispute with sponsors.
Victories came, but it was no longer the Roche of 1987. He signed up with the Belgian sponsor Histor on an earn-as-you-win contract that was worth half the deal he had, and lost, with the Spanish company Fagor.
At the start of this year's Tour Roche's main priority had to be his team leader Claudio Chiappucci's challenge to Miguel Indurain. That has now become as fragile as the mist over the green valleys of the Auvergne.
The Basque still holds a 1min 42sec lead over the Italian, and a fairly firm grip on any threatening moves, plus a banker in Friday's time trial. He beat Chiappucci by more than five minutes in the first time trial, so Friday should be a formality on the road to Sunday's final yellow-jersey show in Paris.
Indurain was ready to admit that yesterday's 212km stage from St Etienne was tough. 'It was not easy to control because riders were attacking all the time. I just watched Chiappucci and my team did a great job,' Indurain said.
This 79th Tour is going to be the fastest. Indurain's average is 39.45kph over the 3,155km so far contested, faster than the 39.1 ridden by his compatriot Pedro Delgado in 1988. 'It is the hardest Tour I have ridden,' Britain's Sean Yates, with nine Tours almost behind him, said.
'No one is taking it easy. Look at the speed and the amount of retirements. Guys who are not going to win are attacking, and every day it's flat out from the gun.'
Bernard Hinault commanded respect, especially with his five victories in the Tour. 'When he said, 'Ease off', people took notice,' the Frenchman Charly Mottet had said before he joined the lengthening casualty list because of sickness. 'Hinault would get angry if the racing was too fast. Indurain is respected too, but he does not seem to worry. He lets it happen.'
Sixty-seven riders have fallen by the wayside, mainly to the exhausting pace.
When Dimitri Zhdanov broke away with 100km remaining, he was joined by the Dutchman Jelle Nijdam and Belgian Peter de Clercq and before they were finally absorbed, De Clercq profited from the escapade by earning his weight in the famed St Nectaire cheese as he won the sprint into Chambon- sur-Lac. Fortunately for the donors it was the diminutive Belgian and not the towering Nijdam.
TOUR DE FRANCE Stage 16 (212km, St Etienne to La Bourboule): 1 S Roche (Carrera, Irl) 5hr 52min 14sec; 2 V Ekimov (Panasonic, Russ) +46sec; 3 J Unzaga (CLAS, Sp) +50; 4 C Chiappucci (Carrera, It) +51; 5 S Rooks (Buckler, Neth); 6 L Jalabert (ONCE, Fr); 7 M Indurain (Banesto, Sp); 8 G Bugno (Gatorade, It) all same time; 9 M Ghirotto (Carrera, It) +55; 10 E Breukink (PDM, Neth) +57; 11 G-J Theunisse (TVM, Neth); 12 P Lino (RMO, Fr); 13 A Hampsten (Motorola, US); 14 R Alcala (PDM, Mex); 15 A Gonzales (Festina, Sp) all s/t. Selected: 32 R Millar (TVM, GB) +1min 28sec; 43 S Kelly (Festina, Irl) +2:06; 64 M Earley (PDM, Irl) +9:24; 115 S Yates (Motorola, GB) +15:41.
Overall: 1 Indurain 79hr 58min 0sec; 2 Chiappucci +1min 42sec; 3 Hampsten +8:07; 4 Lino +9:22; 5 Bugno +10:09; 6 P Delgado (Banesto, Sp) +11:50; 7 Breukink +15:54; 8 G Perini (Carrera, It) +15:56; 9 Roche +17:12; 10 F Vona (GB-MG, It) +19:22; 11 J Heppner (Telekom, Ger) +20:01; 12 Theunisse +20:32; 13 E Boyer (Z, Fr) +20:40; 14 G Rue (Castorama, Fr) +21:29; 15 E Bouwmans (Panasonic, Neth) +22:56. Selected: 17 Millar +24:14; 40 Kelly +1hr 11min 42sec; 76 Earley +2:11:21; 85 Yates +2:19:53.