Heading into their first full season under Bruce Cassidy, expectations were not all that high for the 2017-18 Boston Bruins. Returning to the postseason a year ago, the battered Bruins fell to the Ottawa Senators in six grueling one-goal games.

A combination of aging veterans and inexperienced youngsters make up the core of this season’s Black and Gold. That, mixed in with last year’s playoff return caused many to see the Bruins just as they were when their season ended in April: a mediocre team, one who could land a playoff spot and maybe, just maybe win a playoff round.

Outside of re-signing David Pastrnak, the Bruins had themselves a very quiet offseason. The quietness did nothing to increase the expectations.

With the Bruins at the end of their annual bye week, the expectations have changed. Take a look at any hockey publication’s power rankings, or throw on your favorite hockey podcast. The sentiment is all the same: the Bruins are one of the best teams in the league and are — dare I say it — Stanley Cup contenders.

But how did the Bruins get here? How did they go from a desperate team who was forced to fire the winningest coach in team history to a first-round playoff exit to a now Stanley Cup contender?

Let’s take a look.

Bruce Cassidy:

All you have to do is watch a period of Bruins hockey and you’ll see how much different the team is under Cassidy. They’re quicker in getting pucks out of their own zone, smarter with their decisions in the neutral zone and improving their shot selection in the attacking end.

Looking at the stats under Cassidy will tell you much of the same. He got them playing inspiring hockey en route to an 18-8-1 record in the final 27 games of 2016-17.

That inspiration carried over into this season. Facing injuries to key players from the start of the season, Cassidy has done a tremendous job moving pieces around and finding the right chemistry across the board.

Even more impressive has been the way he’s handled the inexperience of some of his younger players. It was obvious his predecessor, Claude Julien, was more confident in and loyal to his veterans. Cassidy, on the other hand, is not afraid to roll out his young guns when it matters most.

Cassidy has shown he’s not afraid to sit a guy if the situation calls. Players like Jake DeBrusk and Anders Bjork had very impressive responses immediately after returning to the lineup after a few games from the press box. And don’t forget his willingness to ride the hot hand in Anton Khudobin while his franchise goalie Tuukka Rask was battling some rough waters in mid-November.

The best line in hockey:

The aforementioned injuries gave way to Cassidy reuniting the trio of Brad Marchand, Patrice Bergeron and Pastrnak. All that has done is give the Bruins arguably the best line in the National Hockey League.

To date, the trio has allowed just one — yes, just one — five-on-five goal. Marchand and Pastrnak are tied for the team lead in goals with 17, while Bergeron trails his fellow linemates with 16.

What has been most impressive about the line is their puck possession. They’ve given fits to opposing lines every time their skates touch the ice.

We’ve seen over the last few years the chemistry between Marchand and Bergeron. That chemistry has grown with Pastrnak to form one of, if not the best line in hockey.

The youth movement:

The Bruins entered the season expecting top-six minutes from Bjork and DeBrusk. The Bruins also expected Charlie McAvoy to pick up where he left off from last year. Even then, the Bruins expected some of their young rookies to secure spots on the roster and log impactful minutes.

They’ve gotten more than expected out of McAvoy and the rest of the youth movement.

Making his NHL debut in last year’s Stanley Cup Playoffs, McAvoy opened many eyes in and out of Boston. Watching McAvoy shift after shift would lead you to believe that last year’s series with the Senators wasn’t his first rodeo.

Not only has McAvoy picked up where he left off, he’s taking his game to new heights en route to being a likely finalist for the Calder Trophy. McAvoy is growing into a bonafide top defenseman before our very eyes.

DeBrusk has had an up and down season to date but is quietly tied for sixth on the team with 21 points. He’s fit in nicely of late on a line with David Krejci and Ryan Spooner, helping out by producing a solid second line.

A tough fibula injury to Adam McQuaid forced Charlestown native Matt Grzelcyk into the lineup. The former Boston University captain has been nothing short of excellent since being called up from Providence. McQuaid has been cleared to return to the lineup for about two weeks now, but Grzelcyk’s play is keeping the veteran put on Level 9.

The biggest surprise undoubtedly has been Danton Heinen. The 22-year-old has essentially come out of nowhere to become one of the Bruins biggest scoring threats. His 30 points (10 goals, 20 assists) are good for fourth on the team behind Marchand (40), Pastrnak (39) and Bergeron (32).

Four solid lines:

Over the course of the last several seasons, the Bruins have yet to find consistency in their four lines. Whether it was finding the perfect winger for Marchand and Bergeron to finding the right mix of forwards for the second, third and fourth lines, the Bruins haven’t had the luxury of rolling four solid trios.

That hasn’t been the case this year. Cassidy is getting production from each of his four lines, including his bottom six of Heinen, Riley Nash, David Backes, Sean Kuraly, Tim Schaller and Noel Acciari.

For a team that possesses one of the top trios in the NHL, having added depth from the Bruins second, third and fourth lines is an added luxury.