In a group where all four teams remain in play, there is everything to gain and everything to lose.
Few anticipated this scenario when the draw was made many weeks ago at UEFA's headquarters in Nyon, Real Madrid desperately needing a result against group leaders Borussia Monchengladbach to avoid dropping into the Europa League.
Lose to the Germans and Inter Milan beat Shakhtar Donetsk and Zinedine Zidane's side would be out of Europe full stop. They have 90 minutes left to prevent their own Nightmare Before Christmas.
Pressure is mounting on Zidane as it is and that is with their hopes of a spot in the knockout stages of the Champions League still alive, albeit with a pulse getting fainter and fainter.
In the reverse in Germany they were lucky, a 93rd minute Casemiro equaliser depriving an impressive Monchengladbach of three points.
And so now a point separates Monchengladbach in first place and Real Madrid in third. Much like Manchester United on Tuesday night, Real face a winner-takes-all scenario and unlike Ole Gunnar Solskjaer's side, Zidane's squad intend to take it.
Sportsmail breaks down where it can fall down for an out-of-sorts Real.
What exactly do Real need tonight?
It's a fair question, albeit one littered with permutations.
Defeat and, much like Manchester United found out against German opposition, they are into the Europa League. If Real lose and Shakhtar Donetsk lose to Inter Milan, Real finish bottom of Group B.
Los Blancos have emerged from all 24 groups they have been part of during the modern era of the Champions League. A flawless record that is in danger of being tainted.
If Inter do beat the Ukrainians then a point will suffice for Real and Monchengladbach.
So, yeah, there is still plenty to play for.
Dressing room problems
Keeping the peace is far easier when you're winning and in Zidane's first spell in charge, that was pretty much all Real did. Win, win, win, and win some more.
But things are different now. The squad is older, more susceptible to injury, suffering longer streaks of bad form and the old guard are finding it hard to play nice with the emerging youngsters.
Only last week did a report come out in Spanish newspaper Sport that there has been a 'total rupture' in the relationship between Zidane and his players.
It is claimed he has lost faith in Eden Hazard, who suffered another injury setback recently against Alaves, and Martin Odegaard, as well as Brazilian pair Rodrygo and Vinicius Jr.
Zidane is also said not to trust a large part of his current squad to perform on the pitch and is particularly irritated by the attitudes of experienced pair Marcelo and Toni Kroos.
But this is not an isolated report of disharmony this season, far from it.
Back in October, striker Karim Benzema was caught on camera accusing Vinicius of 'playing against us' while talking with Ferland Mendy during their 2-2 draw with Borussia Monchengladbach.
'Don't play with him. On my mother's life, he [Vinicius] is playing against us,' he told French defender Mendy. The full-back was heard replying: 'When he becomes scared, he loses what makes him great.'
Vinicius Jr played down talk of a rift as 'just noise' but it was an episode highlighting the stress bubbling away in the group.
Last month Benzema's agent tore into defender Raphael Varane and said the Frenchman has been 'a shadow of himself'. Again, hardly conducive to incredible team spirit.
More recently, Spanish outlet Diario Sport reported Madrid captain Sergio Ramos launched an attack on Isco and Marcelo for their underwhelming displays when they have figured.
Suffer elimination from the Champions League on Wednesday night and Zidane could have a mutiny on his hands.
Would defeat cost Zidane his job?
It's not a question the Frenchman is entertaining on the eve of their biggest match so far this season and that makes sense.
But while he has credit in the bank from delivering three successive Champions League titles to the Santiago Bernabeu, patience is said to be thinning for Zidane and this current incarnation of the side.
He has remained bullish, insisting he will not resign and instead the power will be with the club depending on how Wednesday night plays out.
'The club will do what they have to do (in the event of not qualifying), as they always do, but I'm only thinking about the game,' Zidane said on Tuesday.
'I'm not thinking about anything else other than winning. I'm focused on tomorrow's game and we know what we have to do. It'll be tough, but we need to be able to walk off the pitch satisfied with our work.
'We know there's another game going on (Inter versus Shakhtar), but we're only thinking about our game. We're up against a tough side who will make life difficult for us.
'We know exactly what the situation is. We want to get the three points and finish top of the group; that's what we're all thinking about. All matches are important but this is a great chance to show what we are as a team.'
Fourth in LaLiga and staring the Europa League in the eye, if things aren't uncomfortable for Zidane right now, defeat would certainly help ramp up the tension.
What about form and injuries?
Well, Zidane must get his side ready with only one fit senior striker - Benzema.
Luka Jovic has tendinosis in the adductor muscles of his right leg while Mariano Diaz remains unavailable with an injury issue of his own.
And so, in a sense, it is Benzema or Bust in attack in what is a must-win game at the Alfredo Di Stefano stadium.
Real are also without Eden Hazard and also without midfielder Martin Odegaard. But, and this is the big one, Ramos could well make a timely return to the side.
'It's the most important match of the season and we're focused only on that,' Brazilian midfielder Casemiro told reporters.
'It's a final and we're going to give it everything. We're not considering any other result than a win. The opponents are top of the table, but we have to understand that it's in our own hands.
'We want to go out there and give our all, we know that it'll be difficult, but if we win we go through'.
Madrid have preferred life on the road of late - the two wins in their last five games all came away from the Spanish capital.
In contrast, Monchengladbach have become rather used to playing at their Borussia Park stadium, something they will need to get over on Wednesday.
They deserved to beat Real in the reverse, only for Casemiro to strike in injury time to make it 2-2, and boss Marco Rose knows his plucky side are on the cusp of history.
'Borussia can manage something historic,' he said.
'That we can eliminate Real Madrid is of no importance for us.
'We are ready for the game and we know our duty. That's why there is no reason to be nervous. We are looking forward to the challenge and want to take advantage of our good position.'
Rose will hope Nico Elvedi can pass a late fitness test but he will have to be without Tony Jantschke, Ramy Bensebaini and Jonas Hofmann.
Why are Monchengladbach such a danger?
Rose's side have made a solid, if unspectacular, start to their latest Bundesliga campaign but they remain one of the most exciting young teams to watch in Germany.
Defeat to Inter Milan last week represented a real disappointment but this young group have developed an impressive big game mentality that will be vital in Spain.
The Foals have a solid record when it comes to picking up results in the really big games, having won three of their last six meetings with German giants Bayern.
They also saw off Roma at home in last season's Europa League and earned a creditable draw with Manchester City during the 2016-17 season.
Games against Real were common during Monchengladbach's glory decade of the 1970s, and they also faced them during the 1980s too. In four meetings, the Bundesliga outfit won once, drew twice and suffered just one defeat.
This is a side packed with goals, too.
Only once in their last seven matches have they scored less than two goals. In total they have netted 22 in seven games, a warning to Real that mistakes will be punished.
Couple that with the fact Real have failed to keep a single clean sheet in their last five home games and it is fair to expect goals when the whistle blows on Wednesday night.