Captain K9 tells the story of Elizabeth Blue and her imaginary superhero show Captain K-9. Elizabeth's mother is the manager at a local public access station, where Elizabeth and her brother Benji spend their time after school and occupy themselves by making a show in an unused part of the station starring Zachary, Elizabeth's dog. When Elizabeth's best friend, Jessica, experiences changes in her homelife, she withdraws. Elizabeth, unable to understand the issues Jessica is having, interprets them as villainous, and reflects that in her show.

New York realtor Bella Castle's divorce is amicable and peaceful. But the news that her ex-husband Mick is going to be a father again throws Bella off course.