Karim Adeyemi scored three goals in the first half as Borussia Dortmund handed Celtic a brutal Champions League reality check, winning 7-1 at home on Tuesday.
Celtic came into the match boasting a 100 percent record in Scotland and Europe this season, but were blown off the park in Germany, Dortmund leading 5-1 at half-time.
By then, Emre Can had opened the scoring from the penalty spot while Adeyemi had already scored three goals and had won another spot kick, converted by Serhou Guirassy.
Adeyemi was a constant threat for the home side, popping up all over the pitch to bamboozle the Celtic defence.
The Germany forward was subbed off three minutes into the second half but Guirassy scored an excellent solo goal and Felix Nmecha netted a late strike.
The victory equalled Dortmund's biggest ever Champions League win while leaving Celtic's ambitions of impressing on the European stage in tatters.
"It was a very special game," said Adeyemi, who was named man of the match.
"The team scored a lot -- every shot went in."
Dortmund coach Nuri Sahin praised Adeyemi but called on the forward to be more consistent.
"Karim simply has everything, he's so strong, but we need to work together to see that on the pitch every three days."
Can told Amazon Prime: "There's no such thing as a perfect game, but it was very, very good.
"This has to be the standard we aim to set."
Celtic captain Callum McGregor told TNT Sports his side were "rattled" by conceding so quickly after they had equalised to make it 1-1 early in the match.
"The quick goal killed us and rattled us a little bit," he added.
"When you give good players time on the ball, they will kill you. And that's what they did."
Domestic league champions in 12 of the past 13 seasons, the European stage has long been the true test of Celtic's quality -- and it has often sent them crashing back to earth.
Both sets of supporters united to sing 'You'll Never Walk Alone' before kickoff, a staple of the pre-match ceremony at Dortmund and Celtic, who share a long fan friendship.
The smoke from the Celtic fans' pre-match pyrotechnics had barely cleared before Dortmund took the lead.
The lightning-fast Jamie Gittens was felled in the box by Kasper Schmeichel, bringing Can to the spot, the captain converting for his first goal in the competition since his Liverpool days in 2017.
Celtic swiftly hit back, Can turning villain when he failed to clear a simple cross, allowing Daizen Maeda to equalise.
Any hopes Celtic had of taking the game to Dortmund were short lived, Adeyemi latching onto a Julian Brandt pass to reclaim the lead.
While his first goal showcased his speed, his second illustrated his power from range as he blasted in to the roof of the net from a tight angle on the left after 29 minutes.
He was at it again 11 minutes later, winning a penalty in a tussle with Arne Engels, which Guirassy slotted home.
With Celtic looking longingly at the clock with half-time approaching, Adeyemi completed his hat-trick, pouncing on a poor clearance and curling past Schmeichel.
Adeyemi was subbed off injured three minutes into the second half but Dortmund continued to turn the screw.
Guirassy scored a second after slaloming through the penalty box and Nmecha adding a seventh with 11 minutes remaining.
The big win means Dortmund finish the night atop the Champions League table while Celtic have fallen into the bottom half, despite a dominant 5-1 win over Bratislava in their opener.
Dortmund sporting director Sebastian Kehl confirmed Adeyemi had "a muscle injury" in the thigh, adding "it's a shame as he's in top form."
"I hope it's not too bad and I'm back on the pitch quickly," Adeyemi said of his injury.
"It doesn't feel to bad, I'm staying positive."
