Crafting the narrative for this season in Scottish football presents an extraordinary challenge. How can a follow-up possibly rival the original?

In the upcoming month, fans will witness an intense three-way showdown for the championship.

No matter the outcome, the conclusion of this season promises to be steeped in excitement and suspense.

A partnership that began three decades ago when O’Neill brought his fellow Northern Irishman from Crewe to Leicester City is set to culminate in what is likely the veteran manager’s swan song.

After a season filled with high stakes and captivating sub-plots, this seems like a fitting end to the saga.

Substitute Kelechi Iheanacho puts Celtic in front with a glancing extra-time header at Hampden

St Mirren striker Mikael Mandron celebrates his stoppage-time equaliser in the semi-final

Striker Daizen Maeda gives Celtic an early lead by charging down Ryan Mullen’s clearance

Lennon, attending in his role as a TV analyst, had a moment of uncertainty, wondering if he and his mentor would indeed meet again next month.

Back to their best in the first half, and deservedly two goals ahead, Celtic turned in the kind of sub-standard second half show which has been so characteristic of them this season.

Trailing by goals from Daizen Maeda and Tony Ralston, St Mirren came out swinging. When Mikael Mandron struck his second of the day in injury time, everything pointed towards the Buddies completing the job in extra-time.

Celtic came again, though, producing an extraordinary salvage act by scoring four times in the first period of extra time.

O’Neill’s substitutes would prove decisive with Kelechi Iheanacho bagging a double either side of Luke McCowan’s strike. Benjamin Nygren completed the scoring. St Mirren just didn’t know what had hit them.

Only those with a heart of stone wouldn’t have felt some sympathy for Grant Tamosevicius as the ball flew past him in those moments.

Only on the bench due to an injury to Shamal George, the teenager was introduced for his debut when Ryan Mullen was forced to come off after 15 minutes.

By then Saints were already trailing and seemingly going down with a whimper. Little did we know how it was to all unfold.

Two months ago, O’Neill chalked up his 1000th match as a manager when Celtic faced VfB Stuttgart.

Back then, Craig McLeish was a little known youth coach with St Mirren. Yet, here the was, leading the Buddies out for a Scottish Cup semi-final in his fourth match in interim charge. Quite a turn of events.

When the teams emerged, Killian Phillips, the St Mirren midfielder, prematurely broke from a walk into a sprint and started heading imaginary balls at the half-way line.

You took it to be a sign of his side’s readiness and hunger to get down to business. Yet after 57 seconds they were behind.

It was a horrendous episode for Mullen, his concentration clearly not where it needed to be from the first whistle.

There was nothing wrong with Miguel Freckleton’s back-pass. The keeper’s touch was heavy and Maeda drove at him.

By the time the Japanese was on top of him, it was already too late. Mullen thumped his kick straight at him and could only watch on on horror as it crossed his line.

For all the concerns arising from an unconvincing display against the same opponent a week ago, Celtic were initially on it.

Arne Engels struck the outside of the post following a lay-off from Seb Tounekti. Mullen looked nervous every time he went near the ball and soon sat on the turf injured.

What an introduction to professional football this was for Tamosevicius, a 17-year-old youth keeper who’d only recently passed his driving test.

Phillips offered a few words of encouragement as he entered the fray. Heaven knows what was going through his mind.

His first involvement was a wayward pass straight to Tounekti. He redeemed himself by holding Nygren’s strike and settled down.

Nygren’s cute reverse pass found Maeda. Tamosevicius threw himself to his right to keep it out.

Half an hour had expired before Saints threatened. Mark O’Hara’s corner was claimed by Liam Donnelly. Viljami Sinisalo’s save was superb.

The Celtic keeper seemed intent on following Mullen’s lead by giving the opposite side a helping hand.

Slack when taking Auston Trusty’s pass-back, Sinisalo smacked his clearance straight at Phillips, just about managing to claw it away from under his bar.

The Finn didn’t learn his lesson. Minutes later, he showed too much of the ball to Jonah Ayunga. He was relieved to see Phillips sweep the ball wide.

Notwithstanding those two scares, Celtic were on top. Yang Hyun-jun struck the bar with a floated header after meeting Kieran Tierney’s deep cross.

Three minutes into added-on time, O’Neill’s side had the second goal their play merited. It was beautifully crafted with Tounekti bobbing and weaving across the penalty box then playing in Yang.

The Korean took a touch then went to shoot only to produce a deft back-heel. Ralston could not have caught his strike better, finding the net via the underside of the bar.

The half-time whistle found Saints’ hopes of a cup double hanging by a thread. Scoring next was non-negotiable. After starting the second half somewhat smarter than had they first, they did just that.

Declan John found space on the left and delivered a searching cross. Mandron had Trusty and Tierney for company yet outmuscled both of them. His looping header back across goal gave Sinisalo no chance.

The Paisley side threw everything they had at it. Tierney took a sore one to prevent Jonah Ayunga converting another John cross.

Celtic became less of a threat in the final third. Tamosevicius did all that was asked of him without being overly extended.

Sinisalo became the busier of the two keepers, confidently punching and catching a string of dangerous deliveries from set-pieces.

It said much about how weary Celtic looked that the Finn was wasting time long before the end of regulation time.

He did well to prevent Mandron claiming the equaliser after he’d blocked an initial effort, but was helpless as the forward levelled seconds into injury time.

With Celtic failing to secure possession, Freckleton’s long ball was helped on by substitute Jake Young. Goal-side of the Celtic central defence, Mandron’s right-foot strike arrowed into the far corner. We were going the distance.

You had to hand it to Celtic, though. Having been pegged back so late on, all the momentum was against them.

They dusted themselves down to put the game beyond Saints with four goals in six minutes in the first period of extra-time.

Iheanacho claimed the first with a glancing header to convert James Forrest’s cross.

Fellow substitute McCowan calmly stroked home the fourth with a left foot finish after the ball broke kindly to him in the box.

Then, after Iheanacho had found the top corner in nonchalant style, Benjamin Nygren claimed his 20th of the season by tapping in Maeda’s low cross.