Steven Mills (Aykroyd) is a widowed scientist who is working on different ways to send radio waves into deep space. Steven accidentally sends a radio wave out of the galaxy to an alien world, which causes a disruption of gravity on her planet. One alien, Celeste (Basinger) is sent to investigate what affected the gravity and how it was done, believing it was an attack. Celeste arrives on Earth by crashing a party hosted by Steven's brother Ron (Lovitz), after which she moves in with Steven. Steven's 13-year-old daughter Jessie (Hannigan) is at first happy that her father has found someone, as her mother died five years previously, but becomes suspicious of Celeste's inexperience with Earth's customs, but she cannot convince her smitten father that something is unusual about Celeste. Along the way, Celeste encounters new experiences, including love.
This film features examples of:
- 90% of Your Brain: Celeste explains that "We use 104% of our brain capacity, as opposed to your 36%".
- Absent-Minded Professor: Dr. Steven Mills manages to send radio waves out of this galaxy and to an alien planet, which causes a disruption of its gravity (though he couldn't have known that there was a planet there), and is completely oblivious to Celeste's ignorance of Earth's customs, which is half this and half Love Makes You Dumb.
- Alien Catnip: Celeste accidentally ends up discovering that caffeine makes her drunk.
- Aliens Love Human Food Steven gets Celeste to try a sandwich. She devours it.
- Boldly Coming: Pretty much the whole plot of the movie is a parody of this.
- Do You Want to Copulate?: Being an alien, Celeste is completely devoid of any hang-ups and inhibitions humans have about sex. This is not helped by the fact that she got her ideas about human interaction from porn videos.
- Fantastic Diet Requirement: Celeste relies on sucking the insides of batteries for energy.
- Hassle-Free Hotwire: Celeste can start "borrowed" cars just by touching ignition locks.
- Ingesting Knowledge: Celeste can open books, lay her arm across the pages, and react to the contents.
- The Internet Is for Porn: A pre-Internet example. When Celeste at first has no idea what sex is and she asks her talking purse, the purse looks up the term in its dictionary, but the appropriate definition has been redacted. Its search for "instructional videos" then leads it to porn.
- Literal-Minded: Celeste, when preparing breakfast, presents Steven and Jessie with the entire menu.
- Magical Girlfriend: Celeste, although she didn't intend to fall in love with Steven.
- Sense Freak: Celeste has this reaction to food and sex. Also, she gets drunk on caffeine.
- Stay with the Aliens: Ron chooses to go to Celeste's home planet, when he sees that the all-female crew of the alien ship looks like his dream woman - Princess Stéphanie of Monaco. It's implied that his job will be to help the aliens understand things like emotions. Given their complete lack of knowledge about such things as sex, it can be assumed that Ron will have a lot of fun teaching them.
- Toplessness from the Back: When Celeste is seducing Steven, her naked back is seen.