A module from the National Aeronautics and Space Administration (NASA) lands on the planet Mars. While it collects soil samples, a family of four extraterrestrials is accidentally sucked inside through a vacuum hose. The module lifts off, returns to Earth, and is taken to a laboratory in the California desert. As federal agents Mitford and Wickett observe scientists opening the module, the aliens reappear. Discharging electrical power from their hands, they escape into the desert, but the youngest is separated and runs onto a street, causing a multiple-vehicle collision. As traffic stops, the alien hides inside a minivan belonging to a single mother, Janet, and her two sons, Michael and Eric, from Illinois. After agent Mitford waves them past the accident, Janet continues driving to their new home in a Los Angeles suburb. Arriving at the house on the top of a hill, Eric wants to ride around the neighborhood in his wheelchair, but Janet insists he unpack first. Unnoticed, the young alien leaves the vehicle and runs into the canyon behind the backyard. In the morning, Eric sees the alien and follows it. However, he loses control of his wheelchair and rolls off a cliff into a lake. The alien dives in and rescues him. Returning to the house, Eric tells his mother and brother that the Mysterious Alien Creature, “MAC,” saved him, but they do not believe him. To prove that Mac exists, Eric plans to capture him. With help from next-door neighbor Debbie, Eric lures Mac inside the house with cups of a soft drink and traps him inside a vacuum cleaner, but the little alien short-circuits it and creates a neighborhood blackout. As Michael arrives, Eric and Debbie free Mac. Believing his brother is telling the truth, Michael announces that Mac caused the automobile accident and the government is looking for him. In the morning, Eric and Michael find a stack of pictures of horses and a furniture store advertisement on the dining room table. Finding Mac in the living room, they see him pointing at a photograph of their own family. Meanwhile, agents Mitford and Wicket track Mac to Eric and Michael’s house. Worried for their new friend’s safety, Eric and Debbie take Mac to a birthday party at a McDonald’s restaurant dressed as a computerized teddy bear. During the party, Mitford and Wicket attempt to capture Mac, but he escapes with help from Eric, Debbie, Michael, and Debbie’s older sister, Courtney. As they drive into the desert, Mac sees horses grazing around a furniture store billboard. Recalling the advertisement and photographs on the dining room table, Eric believes Mac’s family is close by. Inside an abandoned mine, Michael finds Mac’s parents and sister passed out. Reviving them with soft drinks, the group drives the alien family to a nearby shopping plaza. While Michael and Courtney telephone Janet at a gasoline station, Mac and his family wander into a supermarket and startle the customers. The security guard aims his gun at the aliens, but Mac’s father, curious about the weapon, disarms him. When police and sheriff’s deputies arrive, Michael tries to explain that the aliens are not dangerous, but they ignore him. As Eric rolls in his wheelchair to help Mac, an officer accidently fires his gun and begins a shootout. Eric is caught in the crossfire and killed. Mitford and Wicket arrive as the aliens sit around Eric’s body. With their hands held over him, they use their powers to bring Eric back to life. Later, Mac and his family are sworn in as United States citizens at a ceremony. Afterward, the aliens drive to their new home.