The Sharks have had the measure of the Storm for the past three seasons, winning five of their last seven encounters, including the 2016 grand final and the last two matches in Melbourne.
But the Storm made a mockery of that recent record, with the Sharks, minus injured influential skipper Paul Gallen, never able to get a toehold in the contest.
Slater's heroics started when he saved a try shoulder-charging left wing Sosaia Feki into touch in the corner. The fullback was penalised for the illegal tackle — and there will be much discussion in the days ahead about possible further penalties for that misdemeanour.
But it would be a brave man to deny Slater the chance of a fairytale end to his stellar 16-year NRL career with the Storm.
Cronulla's fortunes took another hit when Feki was forced off with
a shoulder injury after being taken out in the air by Suliasi Vunivalu attempting to gather a bomb.
Slater provided the last pass as Brodie Croft scored the opening try midway through the first half.
Slater scored himself after half an hour, dummying and scything past Matt Moylan close to the line.
He then added a second try just before the break, diving on a diagonal grubber while several players were distracted by a stoush in back play that had Andrew Fifita at its centre.
Several sides have staged impressive comebacks in this finals series — Cronulla had to see off one last week as the Panthers came from 18-0 down to level their semifinal at 20-20.
But Melbourne's defence prevailed. Cronulla had not been kept scoreless in the opening 20 minutes of any game this season until last night — they did not get on the board until Luke Lewis crossed for a converted try in the 68th minute.
In the season decider, the Storm will meet the winner of tonight's other preliminary final between the Roosters and Rabbitohs.
Storm 22 (Brodie Croft, Billy Slater 2 tries; Cameron Smith 3 cons, 2 pens)
Sharks 6 (Luke Lewis try; Valentine Holmes con).
Halftime: 20-0.
