Rex, a Floridian with little more than a high school diploma, manages to almost complete the NASA space training program before her falsified resumé is exposed, but later redeems herself by becoming the only hope for the crew using her smarts, heart, and moxie.

Although Rex is a gifted amateur inventor who earned a full scholarship to Georgia Tech, she dropped out following her mother's death to care for her father and now works as a bartender and alligator wrestler.

At her ten-year high school reunion, Rex encounters her childhood friend Toddrick Spencer, who is now a billionaire specializing in space tourism.

Knowing Rex does not meet most of the requirements, her best friend Nadine heavily embellishes her resume without telling her, leading her to be accepted to an astronaut training program at the Johnson Space Center.

However, after she begins flight training the next day, Logan discovers her falsified credentials when Nadine accidentally switches to video call while pretending to be a male reference.

In the epilogue, Rex reveals that she and Violet have become astronauts for Toddrick's company, the two of them and Nadine have started a "space camp" for young girls, and that she is now in a relationship with Logan.

The website's consensus reads: "Never achieving liftoff despite Emma Roberts' peppy performance, Space Cadet flunks out of the school of high-concept comedy.