The boot of Gordon Ross and Nathan Vella's crucial late try clinched a 48-28 win for London Welsh on aggregate to win promotion.
Bristol topped the Championship at the end of the regular season by eight points, and by eight they trailed with time ticking away as their hope was renewed after a try from Adam Hughes. They came tantalising close, dealing with joy and despair throughout.
It just wasn't to be, Vella's score killing them off with Seb Stegmann's breakaway providing the icing on the cake. The Exiles won 21-20 on the night.
What a turn of events for Welsh, whose relegation from the Premiership last season felt like the last time we would see them in England's top flight for some time. Instead they are back at the first time of asking.
So much focus was on Bristol and their international recruitment before the playoffs, coached under Andy Robinson, that there was barely any mention of Welsh winning promotion. They have swooped in at the last minute, built around outstanding defence and a punishing set of forwards, to return to the promised land. Ross was superb in managing the conditions.
The list of Bristol recruits is impressive; Jack Lam, Dwayne Peel, Anthony Perenise, David Lemi. Ryan Jones, the Wales international, has already arrived. Playing in the Championship will not have been on their minds. Agents will no doubt be contacted over the next few days.
The 19-point deficit Bristol faced felt enormous before the heavens opened, but as the rain fell their chances of producing their trademark ambitious wide play were always going to be hindered. The rain never stopped, but they did score three tries.
Emotions were always going to run high for Bristol, given they were bidding goodbye to the Memorial Ground after 94 years of service.
They faced a seemingly impossible task in the wet, trying to overcome a London Welsh pack who outmuscled them in the first leg in Oxford. To make matters more taxing, Welsh also possessed the best defence in the league.
As long as Bristol's scrum kept turning the screw then fortune may favour them. Nicky Robinson's first strike wobbled wide to the left and sadly his second fell the same way in a sign of things to come. The need for all possible points to be taken didn't have to be stressed.
Time was against the home side and it remained scoreless after the first quarter, Welsh's prowess at the breakdown frustrating Bristol continuously.
Ross turned the screw, a slow drop goal drifting through the posts for the game's first points and to make Bristol's losing battle all that more agonising.
The wily old Scot may have thought Welsh's last stint in the Premiership would be his last, but his tactical awareness was perfectly suited to the conditions.
Bristol had to take something into half-time as much for their spirits as the scoreboard. There was no time left to go for the posts with 50 to go, no time for crucial lineouts to miss their target.
Fortunately, Robinson had a trick up his sleeve. A perfectly weighted cross-field kick was won by Andy Short, who shrugged off two tacklers and slid over for Bristol's first try.
Ross had a crucial say, landing a penalty off the woodwork, before Chris Cook's deliberate knock-on had Welsh reduced to 14 men.
Mitch Eadie's try after the break was massive for Bristol, the number eight who came in for Ryan Jones aquaplaning his way over to send the Memorial Ground into rapture. All of a sudden Bristol only trailed by 12. Their hopes felt alive.
An open game would favour Bristol with their high scoring try exploits this season as Welsh showed signs of weakness, Riki Tipuna and Eadie leading the charge.
Robinson simply couldn't keep missing his kicks, with two penalties and two conversions already dropping wide. Another penalty miss was one too many, Robinson's missed tally rising to 13. That sum would have put Bristol into the lead.
The introduction of Adrian Jarvis was inevitable, but it was maybe too late. Ross on the opposite side kept turning the screw, pinning Bristol back into their 22 with aplomb.
Welsh's breakdown play was utterly supreme, isolating Bristol time and again at the ruck much to Tipuna's obvious frustration.
The hosts were growing desperate. Nick Koster's mishandle on a pass was bad enough, but his hashed kick into Welsh hands ended with a penalty from which Ross stretched the gap again to 15. It felt like the moment the match was settled.
Never speak too soon. Ben Mosses busted through the defence and fed Adam Hughes for Bristol's third try, Jarvis converting to cut the gap to eight points with eight minutes to go.
It wouldn't be a nervous final without a call to the TMO, with Welsh quite literally inches short of a try off a flying, not rolling maul, with Vella at the heart of it and grounding the ball by the tiniest of margins. The Memorial Ground was stunned into silence.
Welsh, unfancied but dogged throughout, are returning to England's top flight. Stegmann's try, racing away and hacking a loose pass before grounding the ball over the line, capped it all off in style. Never write off an underdog.
Tries: Short, Eadie, Hughes
For London Welsh:
Tries: Vella, Stegmann
Pens: Ross 2
Drop Goal: Ross
Yellow Card: Cook
Bristol: 15 Jack Wallace, 14 Charlie Amesbury, 13 Jack Tovey, 12 Ben Mosses, 11 Andy Short, 10 Nicky Robinson, 9 Ruki Tipuna (c), 8 Mitch Eadie, 7 Marco Mama, 6 Nick Koster, 5 Glen Townson, 4 Ben Glynn, 3 Gaston Cortes, 2 Ross Johnston, 1 Kyle Traynor
Replacements: 16 Rhys Lawrence, 17 James Hall, 18 Jason Hobson, 19 Ben Skirving, 20 James Grindal, 21 Adrian Jarvis, 22 Adam Hughes
London Welsh: 15 Seb Stegmann, 14 Rhys Crane, 13 Tom May (c), 12 Seb Jewell, 11 Nick Scott, 10 Gordon Ross, 9 Chris Cook, 8 Daniel Browne, 7 Carl Kirwan, 6 Peter Browne, 5 Matt Corker, 4 Will Spencer, 3 Peter Edwards, 2 Nathan Morris, 1 Nathan Trevett
Replacements: 16 Nathan Vella, 17 Tom Bristow, 18 James Tideswell, 19 Ben West, 20 Richard Thorpe, 21 Rob Lewis, 22 Alan Awcock.
Referee: Luke Pearce
by Ben Coles