He brought it for show-and-tell at school and attempted to shrink Cindy to ant size. The device failed to work and Cindy made fun of him, but as they were leaving the classroom, Jimmy gave it a little hit when he hung his backpack (which presumably fixed it), then the device fired from his backpack and shrank Ms. Fowl, leaving her to battle a gigantic worm in her apple.
Later in the movie, when Jimmy was grounded, he shrunk himself with the Shrink Ray to sneak out. On his way down the stairs, he dropped the Shrink Ray on the couch beside his parents. Thinking it was their TV remote, Hugh aimed it at and shrunk the TV, but re-expanded it, then shrunk Judy's head, but restored her. Jimmy jumped down and grabbed the shrink ray, unnoticed by his parents, and was kicked out the mail slot by his dad, accidentally. He then made his way to Retroland, eventually unshrinking himself. During their showdown with King Goobot near the end, Jimmy used the shrink ray to make himslf planet-size, easily blowing away Goobot's ship.
The Shrink Ray made a cameo in the episode, "Party at Neutron's". Cindy found it at the party and
used it to shrink Carl. Minutes later, Jimmy found the tiny Carl holding the Shrink Ray and used it to make him
The Shrink Ray played a bigger role in "The Incredible Shrinking Town". Jimmy tried to shrink a worker at Retroland when he called him short, but it was knocked into a "Hit-the-Gong" by some kids, and the coach knocked it into the gong, creating an explosion that shrunk every human in the town.
Its CGI model was later used for the Neutron Dimensionalizer in the Chrysler Town & Country commercial.