1 key storyline for each Round 2 playoff series

And then there were eight.

With the second round of the playoffs set to begin Saturday night, we outline one key storyline for each series.

Eastern Conference

Philadelphia Flyers (1) vs. New York Islanders (6)

If you love goals, this series isn't for you - this will be a defensive battle for the ages.

The Isles (1.67) and Flyers (1.78) are allowing the second- and third-fewest goals per game, respectively, in the postseason so far. Philadelphia (.937) ranks second in save percentage, with the Islanders (.934) in third.

New York has a substantial advantage in the underlying numbers, though, ranking fourth in expected goals against per 60 minutes at five-on-five, while the Flyers rank 20th out of 24 teams.

With goals at a premium, the obvious X-factors are netminders Carter Hart and Semyon Varlamov. But what could actually make or break this series is special teams. The Islanders' penalty kill (75%) ranks 22nd after struggling against potent Florida and Washington units. The Flyers' power play (10.3%) has been equally poor, ranking 22nd in the playoffs. Meanwhile, the Isles' power play and Flyers' PK have both been average.

Tampa Bay Lightning (2) vs. Boston Bruins (4)

Despite reverting back to the old 1 vs. 8 reseeding playoff format for a year, we still get an Eastern Conference Final-caliber matchup a round earlier than we should. This is a powerhouse meeting between the two best teams in the conference and the past two Presidents' Trophy winners. Tampa and Boston rank first and second in points, respectively, in the NHL over the past three regular seasons.

These two teams last met in the playoffs in 2018, when the Bolts won four straight contests after dropping Game 1 to take the series in five games. Much has changed since, though. Not only did each squad add numerous different role players to fill out the lineup, but each is missing a star player.

Steven Stamkos (undisclosed) has yet to appear in the postseason and remains out to begin Round 2. His return at any point in the series would be massive considering the Lightning haven't generated consistent offense from anyone other than Nikita Kucherov and Brayden Point.

Meanwhile, it's Jaroslav Halak's crease for the Bruins after Tuukka Rask opted out and departed the bubble. Halak was sharp against the Hurricanes, but he owns an .893 save percentage and a 2.93 goals-against average in 10 career games against the Lightning.

Western Conference

Vegas Golden Knights (1) vs. Vancouver Canucks (5)

These teams are built similarly. Both employ a high-powered top-six forward group and a distinctly different bottom-six group, the latter intended to bring grit and simply not be scored upon. Each team also has stellar goaltending, and both Jacob Markstrom and Robin Lehner happen to be unrestricted free agents at season's end.

The biggest discrepancy comes on the blue line, where the Golden Knights have a sizable advantage. Vegas doesn't have anyone as dynamic as Quinn Hughes, but its collective top four featuring Nate Schmidt, Brayden McNabb, Alec Martinez, and Shea Theodore is better than what the Canucks offer.

Vancouver's already beat two of the league's best blue lines, though. It dispatched Minnesota in the qualifier and then took down defending Stanley Cup champion St. Louis in Round 1. Neither of those teams received strong goaltending, though, and Vegas has two capable netminders.

The Canucks will have to work extremely hard to generate their chances against one of the best puck-possession teams in the league, and Elias Pettersson, Bo Horvat, and company will have to continue to capitalize on their opportunities, especially on the power play, as they did against St. Louis. Easier said than done, though.

Colorado Avalanche (2) vs. Dallas Stars (3)

This series showcases a head-to-head matchup between two of the best players in the NHL. One of these players - Nathan MacKinnon - is obvious. The other - Miro Heiskanen - doesn't garner the same attention.

MacKinnon's abilities are well known. He has blazing speed, soft hands, and a lethal shot - a nightmare combination for any defenseman. But if there's one blue-liner left in the playoffs who might have the best chance of slowing him down (other than Victor Hedman, perhaps), it's Heiskanen.

The 21-year-old is a sensational skater, and always seems to be in the right position to break up scoring chances. The Stars have outshot their opponents 99-72 and generated 57.3% of expected goals when Heiskanen's been on the ice at five-on-five in the playoffs. He also leads the team offensively with 12 points in nine games.

If Heiskanen can contain MacKinnon - and maybe even outshine him, though that's a big ask - not only will the Dallas defender start to receive the national recognition he deserves, but the Stars will have a chance to upset the Avalanche. If MacKinnon runs wild, Dallas has no chance.

(Analytics source: Natural Stat Trick)