The film follows the story of Celeste, an extraterrestrial woman who is sent on a secret mission to Earth, after her home planet's gravity is mistakenly disrupted by Steven Mills, a widowed scientist raising his daughter Jessie as a single father.

The screenplay was written by Herschel Weingrod, Timothy Harris, and Jonathan Reynolds, based on an earlier script by Jericho Stone.

She is aided by a device called Bag: an alien tentacle with a single eye and a mind of its own disguised as a designer purse.

Celeste crashes a party hosted by Steven's brother, Ron, and immediately draws attention to herself by making outdated references to TV shows and political slogans under the mistaken belief that they are current (information that had actually taken 92 years to get from Earth to her home world).

Jessie, Steven's 13-year-old daughter, is at first happy that her father has found someone (her mother having died five years previously) but becomes suspicious when she observes Celeste eating the acid out of batteries and pulling eggs out of boiling hot water with her bare hands.

Screenwriter Jericho Stone developed the story under the working title They’re Coming as a drama, an allegory about child abuse.

[5][2] On June 20, 1984, Variety reported that Catalina Production Group was planning to start principal photography on They're Coming in late 1984 for Paramount, naming the screenwriters as Stone and Richard Benner; actresses considered for the lead by Paramount included Bette Midler, Shelley Long, Julie Andrews and Raquel Welch.

Fox considered Cybill Shepherd and Joan Rivers for the role, but ultimately never produced the film, and it went into turnaround to Weintraub Entertainment Group, where the title was changed to Two Kids.

Dan Aykroyd and Kim Basinger performed impressions of Jimmy Durante from The Man Who Came to Dinner singing "Did You Ever Have the Feeling" in My Stepmother Is an Alien.

[2] Roger Ebert wrote, "Basinger gets most of the good comic moments in the movie and does with them what she can, but Benjamin and his writers seem to have run short of invention.

"[11] The Radio Times gave the film three out of five stars, writing, "Fine moments of inspired lunacy jostle with predictably slight comic relief, but Basinger's eager-to-please freshness and verve make this intergalactic muddle impossible to dislike.

Had it actually been told from the perspective of the scientist's daughter, as the title suggests, it might have been more appealing, but instead a predictable, amateurish script shifts the focus elsewhere.

"[14] Writing in People, Peter Travers said that director Richard Benjamin "tries to disguise the threadbare plot with explosions and special-effects hardware", and called the film "a clanking bore, except for Basinger—a potential star still waiting for the vehicle that will let her shine.

"[15] Time Out wrote, "The film offers several entertaining sequences, [but My Stepmother Is an Alien] is marred by cruel and juvenile gags.

[18] Audiences surveyed by CinemaScore gave the film a grade "B−" on scale of A to F.[19] A South Korean branch of Baskin-Robbins released an ice cream flavor called "Mom is an alien" named after the movie.