But this journey really began May 8, when new general manager Pierre Dorion hired Guy Boucher as coach after he had spent parts of three seasons coaching in Switzerland. It was Dorion's first big move since being promoted to the position a month earlier.
Dorion boldly traded center Mika Zibanejad to the New York Rangers in July for center Derick Brassard, but otherwise the roster that missed the playoffs last season arrived at training camp mostly unchanged.
That's when the adversity began, with forward Clarke MacArthur sustaining a concussion on a hit from teammate Patrick Sieloff during training camp on Sept. 25, after he missed nearly all of last season with a concussion. That created a big hole in Ottawa's group of top-nine forwards.
Shortly after the start of the season in October, starting goaltender Craig Anderson's wife, Nicholle, was diagnosed with a rare form of throat cancer; he missed most of December and January to support her during her treatment.
This is how the Boucher era in Ottawa began, but the Senators were able to succeed under his guidance and change the way they played to become a better defensive team and, ultimately, a playoff team.
Here are five reasons the Senators clinched:
1. Erik the Great
Karlsson is the engine that powers the Senators and is the biggest reason they are a playoff team.
The Ottawa captain is a long shot to win the Hart Trophy this season as the player judged most valuable to his team, but that criteria fits Karlsson perfectly. He has 27 points (nine goals, 18 assists) in his past 25 games and has turned it on when the Senators needed him most.
Karlsson's value to the Senators is immeasurable, and Boucher was able to implement a more defensively responsible system in Ottawa because Karlsson bought in so convincingly.
To see Karlsson put up 71 points (17 goals, 54 assists) in 77 games is not surprising, because he has been among the NHL's best offensive defensemen for years. What is surprising is to see him second in the NHL with 201 blocked shots, surpassing his career high of 175 set last season.
The Senators won a game without Karlsson on Thursday, but they don't make the playoffs without his consistent excellence.
Video: DET@OTT: Karlsson goes top shelf from a sharp angle
2. The reliever
Dorion announced Nicholle Anderson's cancer diagnosis on Oct. 29 and, not knowing at that time how often Craig Anderson would be with the team, he traded a fifth-round pick in the 2017 NHL Draft to the Pittsburgh Penguins on Nov. 2 to acquire goaltender Mike Condon.
It was a situation Condon was very familiar with, having replaced the injured Carey Price with the Montreal Canadiens a season earlier.
Anderson played one game between Nov. 30 and Feb. 10. During that time, Condon went 14-8-5 with a .908 save percentage, 2.63 goals-against average and three shutouts. Only five goalies in the NHL had more wins during that span. Though Condon's numbers were not necessarily at an elite level, he did enough to allow the Senators to stay afloat in Anderson's absence.
Anderson has been outstanding since his return, going 12-5-3 with a .928 save percentage and 2.17 GAA since Feb. 11. His save percentage is fifth in the NHL among goaltenders who played at least 10 games in that span.
Video: OTT@WPG: Condon dives to push puck off goal line
3. Boucher lays down the law
The Senators had just returned from a 1-3-0 road trip through Pittsburgh and California on Dec. 13 when they held a practice at Canadian Tire Centre to prepare for a home game against the San Jose Sharks.
That's when Boucher put his stamp on this team.
He laced into the Senators in a very public way, with the media watching from the stands and cameras rolling, and told his team in no uncertain terms they needed to do their jobs.
Ottawa was 16-11-2 at the time but had allowed 2.68 goals against per game, an improvement on the 2.94 it had allowed last season, but still not to Boucher's liking.
From that point on, the Senators are sixth in the NHL in goals against with an average of 2.49 per game, behind only the Washington Capitals (2.21), Anaheim Ducks (2.31), Los Angeles Kings (2.40), Columbus Blue Jackets (2.44) and Edmonton Oilers (2.45).
For a team that finished 26th in goals against last season, that is a drastic improvement.
The Senators kept themselves in the playoff race by avoiding long dry spells.
They have not had a single stretch of three games this season without picking up at least a point in the standings, and lost consecutive games in regulation six times.
Ottawa's longest losing streak of the season came at the worst possible time, going 0-3-2 from March 25 to April 3, but they won 2-0 at home against the Detroit Red Wings on Tuesday and 2-1 in a shootout in Boston on Thursday to clinch their playoff berth.
The Senators have proven all season they have the ability to bounce back from losses and avoid allowing negative momentum to gather steam. That's a good trait to have come playoff time.
Video: OTT@BOS: Anderson makes pad stop to seal SO win
5. Role players come up big
Forward Tom Pyatt spent the past two seasons playing in Switzerland, where Boucher saw him a lot, and he decided to bring him to Ottawa this season. Pyatt has contributed 23 points (nine goals, 14 assists) and played an important role on the penalty kill.
Center Jean-Gabriel Pageau has 30 points (10 goals, 20 assists) and leads Senators forwards in ice time on the penalty kill.
Second-year forward Ryan Dzingel has 31 points (14 goals, 17 assists).
The Senators did not get the production they expected from forwards Brassard, who had 38 points (13 goals, 25 assists), and Bobby Ryan, who had 24 (12 goals, 12 assists). They also missed MacArthur nearly the entire season before he returned to play on Tuesday. But their depth forwards picked up the slack.
Video: DET@OTT: Pyatt slots loose puck past Howard