Even just a few seasons ago, few Germans would have known much about the club from the 50,000-strong town of Heidenheim an der Brenz, located in southern Germany between Munich and Stuttgart.
Two-time Champions League winners Chelsea are a symbol not only of Premier League largesse, but of modern football.
Since taking over in May 2022, Chelsea's American owners are estimated to have spent more than one billion euros ($1.05 billion) on transfers alone.
The magnitude of the unlikely journey is not lost on Heidenheim coach Frank Schmidt, who has been with the club all the way from the non-professional ranks.
In an interview with AFP, he said the idea of welcoming Chelsea to Heidenheim's 15,000-seat Voith Arena on Thursday would have been "unthinkable" just a few months ago.
"But in the end, that is now the case," he said. "We've earned this through our strong debut season in the Bundesliga."
"It will be a special evening that our club and our fans deserve."
17 seasons
Schmidt's story is so intertwined with that of Heidenheim, it is inseparable.
With Heidenheim in the fifth division of German football, Schmidt became coach just after retiring from the club as a player in 2007, agreeing to do so for a two-week period only.
Since then, he has driven the club up the German football pyramid until a first-ever promotion to the top flight before the 2023-24 season.
In the time Schmidt has been in charge at Heidenheim, German giants Bayern Munich have gone through 12 different coaches, while winning 13 Bundesliga titles and two Champions Leagues.
In September 2023, on his 16th anniversary at the club, he became the longest serving coach in Bundesliga history and celebrated with a 4-2 win over Werder Bremen, Heidenheim's debut top-flight victory.
Upon their arrival in the Bundesliga, they were widely tipped for a swift relegation. Schmidt not only kept them up but piloted them to eighth spot and a Europa Conference League placing.
One of the many highlights of their first top-flight season was coming from two goals down to beat Bayern 3-2 at home in April.
With plenty taking notice of Heidenheim's sharp rise, the club's next challenge was to rebuild after several key players were picked off in the summer.
Winger Jan-Niklas Beste, who last season became the club's first player called up for Germany, moved to Benfica.
Tim Kleindienst, who has since made his Germany debut and has two goals in four games, left for Borussia Moenchengladbach.
"It was clear we could not replace these players one-on-one, because they were key players for us. You can see that with Tim Kleindienst, who is now a German international and has played for Germany from the start in the past few games."
Without the money to bring in big name stars, Heidenheim brought in a mix of experienced lower division players and young talents.
One such talent is Bayern loanee Paul Wanner, who has five goals and two assists in all competitions this season.
Still just 18, Wanner has shown so much promise he is already subject to a tug-of-war between Germany and Austria.
While acknowledging the ebbs and flows that come with playing while still a teenager, Schmidt recognises Wanner's quality.
"If his development continues like this and he stays injury free, he can go all the way to the top."
'Clear underdogs'
Heidenheim started the Bundesliga season on fire and were top of the table after two rounds, before falling back to earth and now sit 15th.
In the Conference League however, they are one of six teams, including Chelsea, to have won three from three.
"Chelsea are the absolute favourites and anything other than winning the Conference League would be a huge surprise. They have incredible individual quality and potential (and are) third in the Premier League."
Despite being "clear underdogs", Schmidt said his team can pull off a huge surprise of their own.
"We have to fight back with a strong mentality to have a chance."
