Fremantle's perfect record in Perth this season was under threat in Friday night's blockbuster after the Bulldogs' flooding tactics ensured scores were level at half-time.
But spurred on by 53,524 fans, Fremantle kicked seven goals to one in the second half to secure the 11.12 (78) to 5.11 (41) win.
While key forwards Jye Amiss (0.1 from six disposals) and Pat Voss (1.0 from eight disposals) struggled in the flooded forward line, Treacy used his big frame to take some important contested grabs on the way to 3.2 and eight marks to go with 13 disposals.
Treacy's contested mark above three opponents and subsequent goal early in the final quarter gave Fremantle a 19-point buffer, and more importantly finally broke the spirit of the flood.
It sparked a four-goal burst from the Dockers as they extended their record at Optus Stadium to 11-0 this season.
The win guaranteed a top-two berth and two home finals for Fremantle (18-2), and also set a new club record for wins in a season.
And if Sydney (14-5) lose to St Kilda on Saturday, the Dockers will be crowned minor premiers with three rounds still remaining.
"I thought they threw something different at us," Dockers coach Justin Longmuir said.
"We persisted well, and we understood that that's the way they're probably defending a little bit more now.
"They get numbers back and they hold their numbers. Usually, when we get that much supply versus their opposition, and that much time in forward half, the dam wall eventually broke, and in that last quarter it did, and it put a bit of a gap on them."
The Bulldogs (12-8) face North Melbourne, Carlton and Melbourne in their final three rounds, and they may need to win all three in order to snare a top-six berth and avoid the wildcard round.
Ed Richards (26 disposals, nine clearances, 752m gained) and Tom Liberatore (28 disposals, six clearances) racked up big numbers for the Bulldogs, while Cooper Hynes kicked two goals from 22 disposals in the absence of Aaron Naughton and Cody Weightman.
Dockers star Luke Jackson (26 hitouts, five clearances, 16 disposals, two goals) edged Bulldogs big man Tim English (27 hitouts, one clearance, 15 disposals) in an intriguing ruck battle.
Fremantle won the inside 50m count 25-14 in the first half, but the Bulldogs made the most of their limited chances to enter the half-time break all square.
Only two goals were scored in the third quarter - both to Fremantle - and Bulldogs coach Luke Beveridge cut a frustrated figure due to his team's costly mistakes.
First, Rory Lobb's attempt to square the ball to a teammate in attack went horribly wrong, with the ball flying over Richards' head before Laitham Vandermeer fumbled the bouncing ball.
It allowed Fremantle to swing it all the way into attack for Treacy to take a strong mark and kick a goal.
Then late in the third quarter, Sam Davidson's attempted square kick across the Bulldogs' defence instead landed in the lap of Serong for another easy goal.
"Yeah, spirit breakers," Beveridge said of the mistakes.
"We were coming and it would have been nice. You anticipate a bit of maybe a bit of 'Kennel' time instead of a bit of 'Wharfie' time during those momentum swings.
"But we didn't get it and there were times when we should have made something of those opportunities. It's a shame."
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