Cometh the hour, cometh the legend, clink-clanking his way on to the Hampden pitch, dripping with silverware from one of the most storied Celtic careers and, now, with the prospect of even more. James Forrest - 35 years young in July - helped turn Sunday's madcap Scottish semi-final in Celtic's favour. Evergreen, in every sense.

It turned into an absolute rout in breakneck speed but Celtic were reeling when it went into extra-time, the favourites surrendering a 2-0 lead against a St Mirren side who have looked them in the eye all season and shown zero fear.

It was 2-2 and St Mirren had the momentum, then Forrest started to play. Thirteen league titles, eight Scottish Cups, five League Cups and an incalculable amount of hunger to add to his total. Twenty-six winners' medals and he continues to play with the hunger of a guy who has won nothing.

The goal that broke open the semi was Celtic's third and, though it was nodded home by Kelechi Iheanacho, it was masterminded by Forrest's energy and accuracy and craft in getting to the line and dinking a cross to his striker.

It was the goal that unsettled St Mirren, the goal that facilitated the deluge that came; three more followed in the next three minutes. Forrest was involved in the one that made it 4-2 as well, Luke McCowan driving another stake into St Mirren's heart.

Forrest tried to make things happen - and did. He was aggressive when Celtic had lapsed into timidity. He was full of verve when too many of his team-mates were toiling. He was the spark. He lifted them all and helped carry them through a day that was brutally tough for a long time and then unimaginably easy for the rest of it.

He's a bit-part winger these days and for many other days that went before. He's started seven games this season, 11 the season before and single figures for the two seasons before that. Part of that was influenced by injury but in other part it was down to the arrival of shiny new wingers who haven't been anything like as shiny as Celtic hoped.

Expensive, too. Sebastian Tounekti and Michel-Ange Balikwisha are the latest two but, for years, Forrest has counted them in and counted them back out again - Marian Shved, Luis Palma, Marco Tilio, Nicolas Kuhn, Liel Abada. Some successful, some not, some others who just disappeared into thin air. That is far from a complete list of Celtic's wide men in recent times. Regardless, Forrest remains the constant, the great survivor and the enduring influencer.

How fitting it is that a season that has thrown up more plot twists than Agathe Christie's collected works will have one last thriller to round it all off?

Martin O'Neill versus Neil Lennon in the Scottish Cup final in May. The icon most probably managing Celtic for the very last time against his one-time captain, the player he bought, nurtured, depended on and protected in a glorious era. Back to the future they go.

The temptation in all of this is to say that you couldn't make it up - this meeting of Celtic greats - but we've known for a long time that Scottish football is a place of infinite wonders where anything is possible and believable.

St Mirren did their darndest to change the storyline. Everything is degrees of sweatiness for Celtic these days and this was another deeply uncomfortable afternoon before the floodgates were forced open and St Mirren washed away.

At the break, when leading 2-0, it looked like a rare thing of late was going to happen - a Celtic stroll. They'd taken the lead when St Mirren goalkeeper Ryan Mullen delayed a clearance and was suckered by Daizen Maeda sneaking up on his blindside like a lion sizing up an unsuspecting antelope. The outcome could not have been uglier for St Mirren or prettier for Maeda.

He got them off to a flyer and Celtic looked sharp. They hit the woodwork twice, they had a danger about them for a little while. St Mirren lost Mullen to injury and had to replace him with the only fit option, 17-year-old Grant Tamosevicius, playing in his first senior game. No pressure, son.

St Mirren found plenty in adversity. Tempo, aggression, work-rate. They pushed Celtic back, hustled them and harried them, lived in their face and watched them struggle.

Mikael Mandron made it 2-1 then 2-2 and they deserved it. Celtic were like a boxer on the ropes, ahead on points and hoping to see out the final moments by covering up as best they could. They got caught. Into extra time it went. And into orbit went Forrest's impact.

Lennon was there to see it. Dunfermline will be underdogs in the final but given that his own team have already beaten Hibs, Aberdeen and Falkirk on their way back to Hampden and haven't conceded a goal to any of them, then he can't have witnessed too much to scare him.

The goal-fest was Celtic at their most ruthless but it was unlike them this season. If they could deliver that kind of efficiency and class in front of goal for the reminder of the season then it's conceivable they would motor on to a double.

There'll be huge encouragement in Iheanacho's performance, a dangerous weapon and the best tool Celtic have for the run-in. His two goals were timely, to put it mildly. Winger Maeda, for all his phenomenal work-rate, is not the centre-forward Celtic need right now.

O'Neill has many things to ponder. What an end game awaits. Five league games and one cup final, the last act against one of his greatest leaders of the past.

They shared a television gantry at the end. Laughing and joking, the mood will be a little more intense the next time they meet.