Chelsea have taken great strides forward under Thomas Tuchel, but will that be enough to defeat the perennial winners in Zinedine Zidane's squad?
Wednesday night will reveal all, but how will each team line up?
The hosts will likely look to start in a similar fashion to the first leg, in which they came out of the blocks flying and probably should've scored more than just one.
In regards to the goalkeeping position, it'll be Edouard Mendy once more between the sticks. Kepa Arrizabalaga may be the cup goalkeeper, but it's Mendy who is first choice and will start in this crucial game.
In front of him, expect Antonio Rudiger to have recovered from the knock that saw him sit out the 2-0 Premier League win over Fulham and slot alongside Thiago Silva and Andreas Christensen.
Danish international Christensen has performed exceptionally in the right of the back three, and appears to have ousted club captain Cesar Azpilicueta for that slot.
As for the captain, he will most likely play as a right wing-back just as he did in the first-leg, with Reece James dropping to the bench.
Mateo Kovacic is still nursing his hamstring injury, and whilst Billy Gilmour started at the weekend, it'll be N'Golo Kante and Jorginho in the middle of the park.
Kai Havertz scored a double against Fulham, but with Mason Mount guaranteed a start, and Christian Pulisic scoring against the Spanish giants last week, it could be shoot-out between Havertz and Timo Werner, one that the former RB Leipzig man would win.
He may have missed a guilt-edged chance last week, but Werner's pace and threat in behind could be too important to ignore.
Real Madrid head into this game off the back of a vital 2-0 win over Osasuna in their fight for La Liga title.
Raphael Varane suffered an injury in the game, and joins Daniel Carvajal and Lucas Vazquez on the treatment table.
The good news for Madrid, however, is that captain Sergio Ramos is back and is fit to start.
He'll partner Eder Militao, with Zidane most likely sticking with a back four after his experiment of a back three didn't really pay off in the first leg.
Marcelo is out, but Ferland Mendy returns, with Alvaro Odriozola the front-running candidate to start on the right.
The tried and trusted midfield three of Casemiro, Toni Kroos and Luka Modric will feature again, with a 4-3-3 the way to go for Zidane.
Ahead of them, Eden Hazard may get a start against his former club, with Vinicius Junior moving over to the right.
Goalscorer in the first leg Karim Benzema will start up top, looking for his 29th goal in all competitions this season.