The Mets made a trade with the Expos in 1984 that changed the franchise forever

From 1974 to 1984, the Montreal Expos enjoyed having Gary Carter, a slugging catcher and tremendous defensive backstop, suit up for them every day. During that span, Carter made seven NL All-Star teams, won three Gold Glove Awards and hit a bunch of dingers. He was a star. 

On Dec. 10, 1984 -- 33 years ago today -- Carter began the next phase of his career when Montreal traded him to the Mets for a quartet of players (Hubie Brooks, Mike Fitzgerald, Herm Winningham and Floyd Youmans). It was a big get for the Mets, who leapt from 68 wins in 1983 to 90 in 1984 behind a couple of young studs named Dwight Gooden and Darryl Strawberry. Now New York had its eyes on the postseason, and picking up a star like Carter was a big step toward making that happen. 
Only it didn't, at least not right away. Carter had a strong first season with his new team in 1985, hitting 32 homers, driving in 100 runs and finishing sixth in NL MVP voting. The Mets won 98 games -- but wound up finishing second in the NL East to the 101-61 Cardinals and missing the postseason.
The next season, though, things were different. The '86 Mets, as you may have heard, won the World Series over the Red Sox in one of the most thrilling Fall Classics in baseball history. To nobody's surprise, Carter had a lot to do with that -- and his first big moment came in Game 5 of the NLCS against the Astros: 

Carter hit a walk-off single in the 12th inning, plating Wally Backman and turning the tide of that series. New York won Game 6 and advanced to the World Series, where Carter again had a lot to do with the final outcome. 
Carter hit .276 against Boston, clubbing two huge homers in Game 4 at Fenway Park ...

... and delivering a key eighth-inning sacrifice fly in Game 6 that scored Lee Mazzilli to tie the game at 3:

Two innings later, the Mets were trailing, 5-3, staring at the end of their season. Down to their final out, Carter began the Mets' legendary comeback with a single to left field. Two singles later, he came around the score, and the Mets would win, 6-5, on a fortuitous grounder to first off the bat of Mookie Wilson. 
The Mets followed up that historic game with an 8-5 win in Game 7, putting a bow on New York's second World Series title. Carter, picked up in hopes of making an ascending team a championship team, had done just that -- and celebrated by emphatically jumping into the air after catching strike three from Jesse Orosco for the final out:

Carter would be elected to the Baseball Hall of Fame in 2003 in his sixth year of eligibility, after a career that made him one of the most formidable and well-rounded catchers to ever crouch behind home plate. 
Safe to say that trade worked out rather well for New York.