The film is a flashback to 1962 when Scotty Smalls moves with his mother and stepfather to a new city. The move gave Scotty very little time to make friends before summer. He happens to follow some of the neighborhood kids after school one day and sees them playing baseball. Scotty watches from a distance and is embarrassed when his uncoordinated attempt at throwing the baseball falls short. All the kids consider him a loser except for Benny Rodriguez, the best player on the team, who is a neighborhood star because of his speed. Benny asks Scotty to play with the team so they can have a full roster of nine players. Benny gets the rest of the crew to accept Scotty by having him hold his glove hand in the air, and hitting a fly ball right into Scotty's glove. Through some practice and tips from Benny, Scotty becomes an integral part of the team.
Through out the film, the gang has many misadventures, including getting into a face-off baseball game against an enemy Little League team, winning the game and celebrating by going to a fair and, after illegally chewing tobacco, throw up on some pedestrians while on a ride. On a hot day, the kids go swimming at the local pool, where Squints fakes a drowning in order to kiss a female, (aptly dubbed Wendy Peffercorn) lifeguard he had a crush on. The gang also play only once a year at night, on the 4th of July.
Meanwhile, the team has to avoid hitting home runs in the sandlot because "The Beast", a huge English Mastiff, lives on the other side of the fence and it would be considered suicidal to attempt to retrieve the ball, because The Beast's penchant for eating boys is the stuff of neighborhood legends. One day, Benny hits the "guts" out of the ball, and he believes they can't play for the rest of the day because they don't have a ball. Scotty indicates he has a ball at home but is actually referring to his stepdad's Babe Ruth autographed ball. Scotty is entirely unaware of the Babe's fame and how priceless the ball is as he only recently learned how to play the game. Scotty takes the ball while his stepdad is on a business trip and brings it to the boys at the sandlot. The boys don't notice the signature and continue to play. Scotty himself hits the ball over The Beast's fence in a home run. He suddenly realizes he shouldn't have stolen his stepdad's ball. The situation becomes far worse when the other boys indicate how priceless the ball is. The boys must figure out a way to get the ball back from The Beast without risking their lives, leading to a wild array of ideas.
Benny is able to retrieve the ball from The Beast's yard, but is chased all around the neighborhood by The Beast. Finally, The Beast is defeated when he chases Benny back into his yard and the fence falls on The Beast, trapping him. Scotty and Benny realize "The Beast" is a dog like any other and they lift the fence off their former foe, who thanks them with licks and reveals himself to be a very nice dog. They then return The Beast home to his owner Mr. Mertle (James Earl Jones) -- whom the boys fear almost as much as they feared The Beast. Mr. Mertle is no more a monster than his dog is, and he tells the boys if they'd just asked, he would have gotten the ball for them. The boys also learn that Mr. Mertle is a former professional baseball player who was blinded during a game, and that The Beast's real name is Hercules.
Unfortunately, the ball has already been destroyed by Hercules, but Mr. Mertle instead offers to "trade" Scotty for another ball -- signed by the entire 1927 New York Yankees World Series championship team. Scotty doesn't want to take something valuable from Mr. Mertle, so, knowing that, Mr. Mertle "trades" the autographed ball to Scotty, as long as he came to talk baseball with him once a week, as part of the deal. His stepdad grounds him for a week, but in the concluding voice over, Scotty says that things worked out between them.
The film then shows how all of the Sandlot kids one by one left the sandlot, growing up or moving away. Eventually, only Scotty is left at the sandlot.
In the end of the movie, when it comes to the present, Smalls is a radio commentator for the Los Angeles Dodgers, and Benny, Benny "The Jet" Rodriguez, is a star player -- in the twilight of his career, but steals home in the movie's final moment before flashing a thumbs-up at Smalls in the press box.
The film focuses on themes such as friendship, baseball, heroism, and responsibility.
Benny also filled in when the others were batting.
Song in order of appearance: