When Sean Jablonski first saw a script for the UFO series “Project Blue Book,” it was described to him as “ ‘The X-Files’ meets ‘Mad Men.’ ”

The television writer, who signed on as executive producer and showrunner, was immediately hooked. “I’m 100% a believer,” Mr. Jablonski says, adding that he never forgot the time when, as a 10-year-old in New York City, he spotted something in the sky that defied any explanation he could come up with.

This story originally appeared in The Wall Street Journal.