Paul Lambert remembers the moment the flutters of self-doubt became a knot inside his stomach. It was the summer of 1996 and the Aston Villa manager had been fighting, as a triallist, to win a playing contract at Borussia Dortmund. His deal at Motherwell had expired and he saw his slingshot at the big time. But then came the meeting when he realised that surely it could not happen.

"I walked into the dressing room at the training ground and I've seen all the players," Lambert says. "Dortmund had given all their German players who won Euro 96 time off but they were back ... Jürgen Kohler, Steffen Freund, Andreas Möller, Stefan Reuter, Matthias Sammer.

"I remember thinking: 'No. You're never going to do it.' There was unbelievable self-doubt, that I couldn't handle that company because when I saw the players ... He'd won Serie A, someone had won the World Cup, someone had won the European Championship, the Bundesliga titles ... and I'm coming from Motherwell on a free transfer. I was worth a bottle of Coke. Jesus!"

Lambert would end the season as Dortmund's man of the match in their Champions League final triumph over Juventus, having shackled a bloke called Zinedine Zidane in midfield and set up the opening goal of the 3-1 win for Karl-Heinz Riedle. The Glaswegian had been outstanding in both legs of the semi-final victory over Manchester United and he became the first British player to win the European Cup with a non-British club.

Lambert's whirlwind journey from gun for hire to Dortmund legend feels almost incomprehensible, the sort of thing that features only in comic books, but as the club prepares for its second Champions League final appearance, against Bayern Munich at Wembley on Saturday, it provides insight, context and, above all, sheer enjoyment. "In your own eyes, when you've played in Scotland from St Mirren to Motherwell, you think you're a professional footballer," Lambert says. "But when I went over there, I thought: 'Whoa, this is what you call professional football.' The stadiums were huge, the crowds were massive, the standard was fantastic. That's when I realised I was in big-time football.

"People talk about the German focus and mentality and ask: 'How do they do it?' I understand how they do it. They are just so focused on what they are and they don't really bother too much about outside influences. They focus on what they have to do and they have great self-belief. That was where my career changed."

Lambert had resolved to leave Motherwell in search of a fresh challenge and, like every player, he thought somebody would want him, that he would get something. "You always think, 'I'll be all right,'" he says, but t "Lo and behold, people think you're crap," he adds. Motherwell, under the management of Alex McLeish, wanted him to stay and they thought that he would come back to re-sign for them, possibly with his pride dented.

Lambert turned to a Dutch agent. "He's dead now, God rest him," he says. "He said for me to give him 10 days. Motherwell were nearly in pre-season so he had to find me something. On the 10th day, sure enough, he phoned to say he had two teams for me to trial with. I thought he was going to say somewhere like Azerbaijan or Liechtenstein because I had nowhere to go; nowhere in England, nowhere in Scotland, nothing. But he said PSV Eindhoven and Borussia Dortmund."

Dick Advocaat was the Eindhoven manager and he played Lambert on the right of a four-man midfield, which was not his position. He scored a couple of goals but it did not work out. And so to Dortmund, against whom he had played impressively for Motherwell in the Uefa Cup.

"I had to get a car from Dortmund to Lübeck, where the club was playing a small four-team tournament, with two 45 minutes, one against Lübeck, one against Hamburg," Lambert says. "It was a four-hour journey and I'm sitting in the car with serious doubts in my head. I got to Lübeck, saw a massive yellow and black flag at the airport, fans everywhere and I'm thinking: 'You're out of your depth here.'

"Anyway, I played centre midfield in both games and it went all right. I went back to Dortmund, trained for a few days and then there was another tournament. My first game was against Schalke, who I didn't realise until then were Dortmund's biggest rivals. We lost 3-1. Then we played Borussia Mönchengladbach, I got injured after 20 minutes and went off. I thought: 'That's it.'"

Complicating matters further was Dortmund's high-profile purchase of the Portugal midfielder Paulo Sousa, who had led Juventus to the Champions League. Lambert put his faith in destiny. "Wee Billy Davies [then of Motherwell] was phoning me up, asking, 'Where are you?'" Lambert says. "I said, 'I'm in Dortmund.' He said, 'Big Alec is going mad, asking where are you?' Because I just went. I just took the chance, packed my bags and went."

The gamble paid off. Lambert was granted a contract at Dortmund on the eve of the Bundesliga season and he made his debut on the opening weekend, in the 4-2 defeat at Bayer Leverkusen. He struggled to contain Paulo Sérgio, who scored twice. "They were my fault," Lambert says. "I never knew the language, the terminology on the pitch. Jesus! What the hell was this? But I scored to make it 2-1 and after that my confidence really picked up."

The true turning point came four days later, on his home debut against Fortuna Düsseldorf. "Mr [Ottmar] Hitzfeld said: 'Paul, you've done really fine but obviously we've signed Paulo Sousa for seven million deutschmarks. If he is fit, he will play. If not, you'll play.' Paulo didn't make it, because of his knee, and I had one of those games where I couldn't do anything wrong. It could come off the back of my head and go to one of my team-mates.

"It just snowballed into the next game and the next game. All of a sudden, I felt a part of it and then the crowd took to me and I became a mainstay. You become one of them. You can talk about world-class players and it can be flippant, but not in this case. They were genuine world-class."

Lambert's spell at Dortmund was short – he left for Celtic in November 1997 – but it was impossibly sweet. He learned to speak German, he immersed himself in the club's culture and his recall of every last detail is total. "I haven't a clue where my Champions League medal is because the memory is more important than the actual piece of metal, knowing that I've done it and I've come from Scotland to do it," Lambert says. "I just thought: 'I'm going to go for it.' The trial could have gone belly-up and I'd have had to go back to Motherwell with my tail between my legs, but I took the gamble. I've always seemed to get there the hard way."