It could see the elimination of champions from England, Holland, Russia, Portugal, and Italy, while reigning champion Chelsea has a nerve-wracking away game to negotiate too. The previous Matchdays have provided late drama, superb goals, surprising shocks and stars of the future. Here are some storylines to watch from Matchday Five...
1. Nervous times for English giants. When the draw was made for the group stage, two of English football's biggest spenders might have looked at Matchday Five and seen it as the round when their places in the next phase would be confirmed. For English champion Manchester City, and European champion Chelsea, that is far from the case.
City faces Real Madrid at the Etihad and while the pre-match build-up to their Matchday One clash focused on the Roberto Mancini-Jose Mourinho rivalry, City's one-time flirtation with Cristiano Ronaldo (which may yet be rekindled, with the winger playing in Manchester this week for the first time since leaving United), and the Mario Balotelli sideshow, there is only one story this time around: fail to beat the Spanish champion, and City is out of the Champions League again.
Mancini has said "a miracle" is required for City to progress and he is right; City is winless in four and was fortunate to avoid defeat at home to Borussia Dortmund. And a reminder for any fans going: don't leave early. Three of City's group game matches have hinged on dramatic moments in injury-time (conceding a winner in Madrid, equalizing against Dortmund and failing to get a penalty against Ajax).
Chelsea also faces a tricky trip to Turin with Group E delicately poised. Group leader Shakhtar Donetsk is ahead of Chelsea on goal difference, both with seven points, with Juventus on six points. Tuesday's Chelsea-Juventus clash has a knock-out feel to it: if Juventus win, then Chelsea's fate will be out of its hands on Matchday Six; a draw or away win and the reigning champion should be fine.
Shakhtar must also beware. It has been on top for almost all of its games so far and still is at risk of not qualifying. For a team that dominated Chelsea as it did at Stamford Bridge -- and still ended up losing -- it would be a loss to the competition were it not to reach the Round of 16.
2. All eyes on Zlatan. 'The world at his feet' is how L'Equipe headlined Zlatan Ibrahimovic's four-goal haul as he inspired Sweden to a 4-2 win over England last week.
'We've never seen a player like him', wrote Gazzetta dello Sport while Bild added: 'No one will ever forget that goal', referring to the 30-yard overhead kick that sealed the victory. Given that Ibrahimovic has just served a two-match suspension in France for a kung-fu kick on Saint-Etienne goalkeeper Stephane Ruffier, Paris Saint-Germain's game at Dynamo Kiev is our first opportunity to see him in action since that victory, after which he was asked, "What goes through your head in that moment?" "Magic, just magic," he replied.
On Matchday Four, Zlatan set up all four goals in PSG's 4-0 demolition of Dinamo Zagreb. With 18 goals in 19 games this season, and a win ratio of 68 per cent, Zlatan is on a streak right now -- and PSG needs him now. In Zlatan's absence, PSG drew to Montpellier and lost to a Rennes team that played with nine men for over half an hour.
3. Can Celtic make Barcelona win count? On the one hand, it was the biggest shock in Champions League group history: Celtic, from the country with a UEFA co-efficient ranking of 26, beating Barcelona, from top-ranked Spain, in a dramatic night on Matchday Four (as the Irish Examiner's Miguel Delaney wrote, even more impressive than BATE Borisov, from 20th-ranked Belorussia, beating Bayern Munich, from third-placed Germany, earlier this season). Celtic's team cost €7.2m to put together and contained five players signed on a free transfer; Barcelona's bench, on the other hand, was worth over €125m.
Then again, was it such a surprise? Celtic had played a similar tactic on Matchday Three at Camp Nou, and was only undone by a last-minute winter from Jordi Alba. As many Celtic fans have since pointed out, over 180 minutes against Barcelona, it was only behind for one of them. Barcelona was dignified in defeat, with coach Tito Vilanova suggesting that his team would win a game like that 95 percent of the time. "We don't need a Plan B, we just need to make Plan A work better," said defender Dani Alves.
But Celtic's famous win, which has been ranked just behind the 1967 European Cup final victory over Inter Milan in its history, will be meaningless if the team fails to qualify from Group G. At the moment Celtic is three points clear of third-placed Benfica, to whom it travels on this week. A draw will as good as secure a place in the Round of 16, while a defeat would leave it needing to win the home tie against Spartak Moscow on Matchday Six. It could be a nervy night in Lisbon, though it has been lost on no-one that the Portuguese capital was also the scene for that 1967 success. The Lisbon Lions have become Lennon's Lions.
4. Giroud wary of Montpellier threat. With five goals in his last six Arsenal appearances, it would seem that Olivier Giroud is getting used to his new teammates. As he put it in an interview with Le Grande Stade: "I've never lost my confidence, never doubted. We're getting to know each other as players now, and the more we play together, the better it is. I've been getting used to the rhythm of games, it's more physical than France, but that's not a problem."
Giroud is likely to start for an Arsenal side needing to beat his former club Montpellier to keep its Group B fate in its own hands. Though the French champion is bottom of the pile and unable to progress, Giroud has warned that it can cause problems: "They have nothing to lose now they are already eliminated and you have to be careful, as that's when some teams are at their most dangerous: the pressure is off them. They didn't become champions of France with no talent."
5. All to play for in Group H. It is the easiest group, in that Manchester United is the only team in the competition with a 100 percent record, yet also the tightest, given that one point separates the remaining three teams. Perhaps Sporting Braga, currently bottom on three points, has the slight advantage: it doesn't have to play United again, and is home to Galatasaray on Matchday Six. But so far, home form has meant nothing in the three-way mini-league: United is the only team to have won at home in the group. Whoever makes it -- and I am tipping Braga at this stage -- will be the team most group winners will want to face in the Round of 16.