The film follows two young boys who accidentally release a horde of demons from their backyard through a large hole in the ground.

When he wakes, he finds that in his backyard, workers have cut down the same tree from his nightmare and unearthed a large geode which is later found by Glen and his older friend Terry.

While Al throws a party, Terry and Glen break open the geode after discovering it has left strange writing on a notepad, and read the incantations aloud.

The next day, Terry brings a heavy metal album to Glen's house with lyrics based on "The Dark Book".

That night, a swarm of moths shatter Glen's bedroom window, and Angus' corpse is found in Terry's bed.

After returning to the house, Al volunteers to inspect the yard, but the others see it swarming with small demons and call her back.

The first draft of the script was written by Michael Nankin when he was unemployed and recently divorced, basing it on "the nastiest thoughts from [his] childhood".

[5][6] Glen and Al were depicted as being more mischievous than shown in the final film, and the demons spread to the rest of the town, where they would drag neighbors out into the streets to kill them.

[5] The house was a real home in Toronto, but an exterior exit had to be constructed for the production, featuring bricks, stairs, and a bug zapper.

Beyond the backyard was a new housing development, so the crew erected a fence around the yard to block the construction vehicles and workers from view.

[5] The effects used to create the demons included stop-motion animation as well as forced perspective, wherein actors wearing rubber suits were made to look minuscule by their position relative to the camera and to the human characters.

Randall William Cook, the film's special effects supervisor, used his hand and eye to create the scene where an eyeball appears in Glen's palm.

[9] Michael Wilmington of the Los Angeles Times, in comparing it to the works of Steven Spielberg and Stephen King, wrote, "whatever minor triumphs it dredges up, is too hopelessly copycat".

[13] In 1992 a sequel was released entitled The Gate II: Trespassers Louis Tripp reprises his role as Terry, but was the only original cast member to return.

[15] In 2020, Louis Tripp worked with Australian musician KidCrusher on a music and video collaboration entitled Sacrifice, reprising his role as Terry for a tribute to The Gate.