One was fighting for the league title. The other was battling relegation. On an astonishing night in Paisley, St Mirren turned Celtic's world upside down, and changed the face of Scottish football.
St Mirren did not just beat Celtic on 24 March, 2010, they demolished their title hopes, ended one man's Parkhead dream, and gave another the opportunity to "bring the thunder back". The Paisley side also brought themselves from the brink when all appeared lost.
Here BBC Scotland takes a look back to 10 years ago today and St Mirren's first ever Scottish Premiership win over Celtic.
St Mirren were at their lowest ebb. They sat just two points above bottom-of-the-table Falkirk, had had not won in 11 league games and triumphed just three times since moving into their new stadium.
But there was more. Three days before Celtic's visit, they suffered League Cup final heartbreak. Underdogs against Rangers, they had dominated the first half before the Ibrox club were first reduced to 10, then nine men. St Mirren just had to find that one goal but instead it was Kenny Miller who scored late on to clinch the trophy for Rangers.
"It will live with the players for a long time," said manager Gus MacPherson afterwards. "What a fantastic opportunity; it's a sore one for us, but we've got to take it on the chin."
Somehow, MacPherson had to get his side up for the visit of Celtic. If they couldn't score against nine-man Rangers - having netted only once in their four before that - how were they meant to do so against a Celtic side who had conceded just once in four games?
This was a Celtic in desperate need of the points, too. They sat 10 points behind Rangers having already played a game more. To keep alive any faint hopes of winning the title, defeat was unthinkable.
They had not lost a league match in Paisley for 21 years and few predicted that run would end on that night.
"Of course, if there's a disappointment, you look for a reaction," explained MacPherson. "But we told the players just to use it as their motivation and it was there for everyone to see."
'We want five'
St Mirren had upset the odds against Celtic a year earlier. Just a week after taking a 7-0 thumping in Glasgow, Billy Mehmet's penalty knocked Gordon Strachan's side out of the Scottish Cup.
The Paisley club eclipsed that in 2010 though. On a remarkable night, Andy Dorman and Steven Thomson stunned Tony Mowbray's side with doubles to hand out a 4-0 hammering.
Celtic had been to Manchester United, Barcelona and AC Milan on Champions League duty that term and suffered nothing like it. It was their biggest loss since the night Artmedia Bratislava stunned Strachan in his first match in charge.
The visitors finished the game with six attackers on the park but still they couldn't score, never mind stop the glut of goals at the other end. They instead played out the final few minutes to taunts of "we want five" from the home support.
A shell-shocked Mowbray said afterwards: "We maybe need to try to play slightly differently away from home. We had six strikers on the pitch at the death, but it left some young defenders exposed to an energetic, decent team."
The repercussions would be huge.
On the same night, Rangers' dream of a domestic treble was ended, with a last-minute David Robertson goal giving Dundee United a Scottish Cup quarter-final replay win at Tannadice.
United would go on to win that trophy, as Celtic's own hopes of securing silverware were killed by a 2-0 loss to Ross County in the semi-final at Hampden.
By that time, Neil Lennon was in charge, promising the Celtic support he would "bring the thunder back". Within 24 hours of the St Mirren hammering, Mowbray had been sacked, along with coaches Mark Venus and Peter Grant. He had won 17 of his 30 matches in charge.
For St Mirren, they felt it overshadowed their win.
"It's the nature of the game," was MacPherson's forthright answer when asked about Mowbray's dismissal. "I was asked if I had any sympathy for Tony. I'm sure Tony wouldn't want any sympathy. That's just the way it is in this industry."
For St Mirren, they would only win one of their remaining eight matches after their Celtic heroics - a 1-0 success over Kilmarnock. Yet, they still finished 10th, with Kilmarnock and relegated Falkirk unable to catch them.
"What they've had to go through in the last few months has been testing for them," said MacPherson. "And they came through it with a fantastic performance."