Mason painting a portrait of his pregnant neighbour Marisa Esteval
Frank and Faye Riley (respectively Hume Cronyn and Jessica Tandy), an elderly couple who run an apartment building and café in the run-down East Village neighborhood, come under threat by a nearby property development. The development manager sends a hoodlum named Carlos and his gang of thugs to bribe the couple and their tenants to move out. When the tenants resist, they punch through artist Mason Baylor's (Dennis Boutsikaris) door, intimidate pregnant single mother Marisa Esteval (Elizabeth Peña) and break retired boxer Harry Knoble's (Frank McRae) jar of tiles. After Frank Riley refuses to move, Carlos vandalizes the café. With this assault and Faye's dementia growing, Frank contemplates giving in.
Things look bleak until the appearance of a pair of flying, living machines descend into the Rileys' apartment that evening, repairing many of the items that were broken. The two extraterrestrials take up residence in the shed at the top of the apartment building, and are dubbed "The Fix-Its" by the residents of the building. Carlos comes back to threaten the tenants once again, but the Fix-Its lure him to the top of the building and into the shed where they scare him away.
Faye and Marisa learn that the "female" Fix-It is pregnant. After consuming plenty of metal and electrical objects, it gives birth to three baby Fix-Its, although one of them is stillborn. Faye buries the stillborn in a flowerpot the next day, but then Harry digs it up, takes it back to his apartment and succeeds in reviving it. Frank and Faye see a boost of business in the café from the demolition crew, while the Fix-Its help in the kitchen.
With Carlos unable to prove the existence of the Fix-Its that had been foiling their plans, Lacey, the development manager, is furious with the delays in evicting the tenants and moves to replace him. Desperate to see the job done and growing more unstable, Carlos breaks into the building's basement to sabotage the building's pipework and electricity, and badly damages the "father" machine in the process. After Harry throws him out, the tenants discover the Fix-It children are missing and go searching for them in the city while Faye stays behind with the "mother" machine as it fixes the "father". When the father machine is repaired, the now-wary Fix-It parents leave to seek out their offspring, and after finding them with Harry, the machine family departs from the planet.
Tired of the delays, Lacey sends a professional arsonist to burn down the building in a staged "accidental fire". Carlos discovers the plan and in a rage sabotages the arson to make the entire building explode, only to then discover that Faye is still in the building. While the arsonist flees, Carlos unsuccessfully attempts to pose as her late son Bobby to get her to leave, but succeeds in rescuing her as the fire spreads. The tenants then return to find the blazing apartment block collapsing, and Faye being loaded into an ambulance.
By the next morning, the apartment block has been reduced to a smoldering wreck. To Lacey's fury, construction is still unable to continue as Harry, sitting dejected on the steps, refuses to leave. Harry is then greeted by the mechanical family later that night, who have recruited countless other Fix-Its for repairs. By the next morning the entire building has been seamlessly restored to brand new condition, forever ending Lacey's demolition plans and resulting in his termination by his employers. Mason and Marisa settle into a relationship, while Carlos has ironically started a friendship with the Rileys, with Faye finally having come to accept her real son's passing. The story then rolls on to an undisclosed period some years later in the future, revealing that skyscraper developments have eventually been built, but this time flanking either side of the tiny apartment building, with Frank's café now doing a roaring trade as a result of the new employment brought into the area.