In the foggy woods of New England, three egg poachers, Matt, Sam, and Burt, are hunting nightingale eggs. When Sam spots what he thinks is a meteor crash nearby, he decides to investigate. He finds a red glowing cave with a stash of large eggs, and in fear he smashing one, he destroys it. Before he can finish, an unseen entity pulls him away, leaving one egg intact. The mysterious being seeks revenge for the destruction, first killing the other hunters and then moving on to the members of a pop band led by the arrogant Rick, who are on a weekend camping trip. The band is joined by Laura, a girl Rick met and invited, not expecting her to come along. Sharon, a member of the band and Rick's girlfriend, is jealous of Laura. Cathy and Tracy are also part of the group.
Tommy is a young boy living in a secluded house with his mother Molly and his short-tempered uncle Bill. He also discovers the cave and brings the remaining egg home, where it hatches. The bald-headed, shaggy-bodied creature from the egg grows rapidly until it is as large as Tommy. Tommy nicknames it "Trumpy" because it has a short, trumpet-like trunk. Trumpy and Tommy quickly become playmates. At one point, Tommy asks Trumpy where he is from, and Trumpy hints at a star map that features the Big Dipper prominently. It also becomes clear that Trumpy has telekinetic powers, flinging various objects around Tommy's room. While the mother alien continues to search for her missing offspring, the pop band stops at Tommy's house for medical care after Laura encounters the alien mother and falls off a cliff, later dying in the house. A strange dot formation (similar to the Big Dipper) is later seen on her forehead.
Rick's friend Brian and uncle Bill go to a nearby ranger station to use the radio. There they encounter the alien mother, as well as the body of a second poacher, which bears the same dot pattern. The alien kills Brian while Bill flees to the cabin. The alien arrives first, however, and kills Tracy in the band's camper. Tommy witnesses the attack through his telescope. The survivors decide to hole up in the cabin until the next day.
Trumpy's mother sneaks into the house and kills Cathy while she is taking a shower. The survivors attempt to come to her rescue when they hear Cathy screaming, and Bill manages to wound the alien with a wild shot. Bill and Rick want to catch Trumpy's mother before she can escape, so Rick takes a rifle and they go in pursuit.
Immediately after they leave, Trumpy appears, scaring Molly and Sharon. Molly grabs a rifle and aims to shoot Trumpy, but Tommy protects his alien friend and hustles Trumpy into the woods. Molly and Sharon give chase, searching for Tommy, Rick, and Bill in the gloom.
Eventually Trumpy and his mother reunite briefly before Rick and Bill find them. The alien mother attacks Bill, who shoots her once before being killed. Trumpy and Tommy disappear into the woods to say their goodbyes before Tommy reunites with his mother Molly, Sharon, and Rick. The film ends with Trumpy moving deeper into the woods as Tommy, Molly, Sharon, and Rick head back to the cabin.
