Especially when she sips French onion soup at a restaurant that was featured in the Julia Roberts movie “Something to Talk About,” Michelle Freenor is an irrepressible tour guide.

She rattles off the history of Methodism in this city, as well as tidbits about William T. Sherman’s March to the Sea. She discusses the canopy of Spanish moss that hangs above Savannah’s streets, whether “Jingle Bells” was actually composed here, and just how haunted one of the country’s largest historic landmark districts might be.

But Ms. Freenor has also emerged in recent weeks in a new role: plaintiff in a federal lawsuit that could reshape Savannah’s lucrative and potent tourism industry. Backed by a nonprofit law firm with libertarian leanings, Ms. Freenor and three others, including her husband, are challenging the Savannah ordinance that requires tour guides to hold licenses and pass regular academic and medical examinations.