The Champions League returns this Tuesday and Wednesday at the Elite Eight stage, which is where things really took off last season. We saw heavy favourites Manchester City and Barcelona upset and marveled at last-minute heroics and controversy, including the Real Madrid penalty at the Bernabeu that sent Juve keeper Gigi Buffon into a white-hot rage before the referee used a different color -- red -- to send him off.

Brace yourself: The stage is set once again. Here's everything you need to know, from the superheroes and the sidekicks who can decide the ties.

Jump to: Spurs-Man City | Man United-Barca | Ajax-Juve

Liverpool vs. Porto

WHEN: Tuesday 4/9, 3 p.m. ET / 7 p.m. GMT

THE BACKSTORY: Liverpool reached the final last year, where they lost to Real Madrid, and they have reloaded in goal with keeper Alisson and in midfield, with the likes of Fabinho and Naby Keita to give the side more defensive grit to go with their free-flowing attack. Liverpool are involved in an energy-sapping title chase with Manchester City in the Premier League, but the European Cup, a trophy they've won five times, remains special at Anfield.

Porto are also embroiled in a tight title race domestically, too, but sailed through the group stages and came from behind to knockout Roma in the Round of 16. Sergio Conceicao's team is incredibly disciplined and rarely makes mistakes. Plus, they're not lacking in motivation after being beaten 5-0 at home by Liverpool a year ago.

HOW THEY MATCH UP: Liverpool's attack is a little less high-octane in nature than it was last year but Jurgen Klopp's side has gained the kind of defensive solidity that helps in knockout competitions. It bodes well against a team that loves to punish mistakes even when they're outplayed.

LIVERPOOL'S STAR TO WATCH: Mohamed Salah. Relative to last season, it may be an off year but it's the kind of off year most players can only dream about.

PORTO'S STAR TO WATCH: Eder Militao. The much-hyped young defender has already been snapped up by Real Madrid for next season and will be kept busy by Liverpool's attack.

X FACTOR FOR LIVERPOOL: Trent Alexander-Arnold. Against packed defenses, his runs from right-back and balls into the box are critical to stretch any opponent.

X FACTOR FOR PORTO: Iker Casillas. The Spanish keeper has plenty of Champions League experience and the ability to remain unfazed even at Anfield. If he can muster the old magic and return unscathed, Porto have a shot.

LIVERPOOL NEED TO ... be patient and not get caught out on the counter. The away win at Bayern, their first away from Anfield after five straight European defeats, suggests they don't need to nail this straight away.

PORTO NEED TO ... ignore the Anfield European night cauldron and focus on keeping it tight and picking their spots.

FIRST LEG PREDICTION: Liverpool, 2-0. An early goal will send Liverpool to a straightforward first leg victory.

Tottenham Hotspur vs. Manchester City

WHEN: Tuesday 4/9, 3 p.m. ET / 7 p.m. GMT

THE BACKSTORY: Tottenham threatened to make the Premier League a three-horse title race for much of the season, grinding out results even during a rash of injuries. Yet they've dropped off in the past month. This is a team micromanaged by Mauricio Pochettino and built in his image. Both Real Madrid and Manchester United were heavily linked with the manager, but he's going nowhere: he has unfinished business at Tottenham.

Pep Guardiola has already won the European Cup twice at Barcelona, but has been knocked out in the semifinals on all four occasions he's reached that stage (once with Barca, three times with Bayern) since winning the trophy in 2010-11. On paper, City are the most formidable side in the competition, and this is the one trophy that's missing since the Abu Dhabi takeover in 2009.

HOW THEY MATCH UP: The fact that Spurs will be playing at their brand-new stadium means we're in uncharted territory: will the crowd still be getting comfortable or is this their chance to turn their ground into a fortress? City, meanwhile, can either out-possess you or go for the jugular with blisteringly quick runs from out wide and rapid build-up through the middle. Choose your poison, but Pochettino's press has given City problems in the past.

TOTTENHAM'S STAR TO WATCH, TOTTENHAM: Harry Kane. Perpetually linked to a move elsewhere, the center-forward -- voted the world's No. 1 striker in the FC 100 -- Kane is that rarest of rarities: a fan who happens to play for the club he supported as a kid.

MAN CITY'S STAR TO WATCH: Raheem Sterling. He's come into own this season in terms of both scoring and assists.

X FACTOR, TOTTENHAM: Christian Eriksen. He has struggled of late, but his free kicks make him a continuous threat, as does his ability to create out of nothing.

X FACTOR, MANCHESTER CITY: Bernardo Silva. He's arguably City's most dependable central midfielder.

TOTTENHAM NEED TO ... stay narrow against City's pacy wingers Sterling and Leroy Sane, exploit set pieces and disrupt City's midfield.

MANCHESTER CITY NEED TO ... play their game, unleash the speedy wingers to stretch the pitch horizontally and avoid lapses in concentration at the back.

FIRST LEG PREDICTION: 2-2. The new stadium effect, coupled with Pochettino's willingness to go for broke, will lead to goals.

Manchester United vs. Barcelona

WHEN: Wednesday 4/10, 3 p.m. ET / 7 p.m. GMT

THE BACKSTORY: Ole Gunnar Solskjaer, who replaced Jose Mourinho as manager in December, was meant to be an interim boss who would buy Manchester United enough time to find a viable long-term replacement. Except the Norwegian has lifted the team to the point that he became the long-term replacement, getting appointed on a permanent basis in late March. Top of his to-do list is getting United back in the Champions League next year -- put simply, that will require a top-four finish in the Premier League or winning this year's competition -- and ready for the inevitable rebuild, so this tie is a case of playing with house money.

Barcelona are on track for a Treble, but in many ways it feels like an understated season. There is more solidity to this side, but relative to previous campaigns, there's more of a dependence on Lionel Messi than ever as coach Ernesto Valverde is struggling to wean the team off its best player.

HOW THEY MATCH UP: Manchester United have plenty of pace up front and Barcelona have to be careful not to get caught out. On the flipside, except for Paul Pogba, United don't have much in the way of creativity in midfield. And there is the perennial problem of dealing with Messi, which United's back four don't appear well equipped to do.

MAN UNITED'S STAR TO WATCH: Pogba. He was signed to be a difference-maker in precisely these sort of game. Will he step up?

BARCELONA'S STAR TO WATCH: Lionel Messi. A bit of a no-brainer here. Goal scorers are supposed to decline in their 30s, but he's once again averaging more than a goal a game.

X FACTOR, MANCHESTER UNITED: David De Gea. The keeper's reputation is higher in England than in his native Spain, where he has made some high-profile errors for the national team. A shutdown performance would prove some detractors wrong.

X FACTOR, BARCELONA: Jordi Alba. Back to his very best this season, the left-back can be a devastating attacking option down the oppositions' right flank, where United often look vulnerable.

MANCHESTER UNITED NEED TO ... shut down Messi as best they can, exploit the space when Barcelona attack and capitalise on their edge on set pieces.

BARCELONA NEED TO ... stay alert defensively and exploit their technical edge, because if it turns into an athletic contest, United will have the advantage.

FIRST LEG PREDICTION: 1-1. Under Valverde, Barca are content with taking their time to finish United off in the second leg.

Ajax vs. Juventus

WHEN: Wednesday 4/10, 3 p.m. ET / 7 p.m. GMT

THE BACKSTORY: After losing the final twice in the past four years, Juventus went all-in over the summer, signing Cristiano Ronaldo from Real Madrid for a package that will cost them nearly $400 million in fees and salary over four years, but the five-time Ballon d'Or winner is a doubt for the first leg.

Ajax have a glorious past -- they've been European champions four times -- but haven't advanced this far in the competition since 2002-03. With a heady mix of youth and experience, they're the romantic choice, having already knocked out defending champion Real Madrid in the last round.

HOW THEY MATCH UP: Juventus are shape-shifters who can either take the game to the opposition or grind it out and hit you on the counter. Expect the latter in this game against Ajax's tidy pass-and-move approach.

AJAX'S STAR TO WATCH: Frenkie de Jong. The 21-year-old all-arounder has already pledged his future to Barcelona and offers plenty of drive, creativity and charisma in the middle of the park.

JUVE'S STAR TO WATCH: Blaise Matuidi. With Ronaldo a doubt, the French World Cup winner packs a physical punch and is critical in maintaining balance between defence and attack.

X FACTOR, AJAX: Donny van de Beek. Difficult to track when he ghosts into the front line, he's best placed to catch Juve off-guard.

X FACTOR, JUVENTUS: Federico Bernardeschi. When Juve need a spark, he's increasingly been the one to provide it and is unafraid to take risks.

AJAX NEEDS TO ... Exploit the space behind the full-backs, not be wasteful in midfield and, above all, play with no fear.

JUVENTUS NEEDS TO ... Remember it's a two-legged game, but not dig themselves a hole like they did against Atletico Madrid, either. Attack can be a form of defence too.

PREDICTION: 1-1. Juve's experience and firepower should be enough to see them through.