A broad brushstroke outline of May 2018's "Star Wars" prequel, "Solo: A Star Wars Story" names major characters and goes some way to explaining their presence.

Since casting began in 2016, we've had a pretty good idea of who'll be in "Star Wars" character backstory spin-off "Solo: A Star Wars Story."

Now Disney and Lucasfilm are starting to spell out why they're involved.

The official synopsis for the May 2018 movie, published January 16, indicates which three main characters are the most pertinent -- or, at least, which plot points Disney and Lucasfilm are encouraging fans to think about.

"Board the Millennium Falcon and journey to a galaxy far, far away in Solo: A Star Wars Story, an all-new adventure with the most beloved scoundrel in the galaxy," the StarWars.com synopsis reads.

"Through a series of daring escapades deep within a dark and dangerous criminal underworld, Han Solo meets his mighty future copilot Chewbacca and encounters the notorious gambler Lando Calrissian, in a journey that will set the course of one of the Star Wars saga's most unlikely heroes."

For the first time, Alden Ehrenreich plays a younger version of Han Solo, originally portrayed by Harrison Ford in "Star Wars" (1977), "The Empire Strikes Back" (1980), "Return of the Jedi" (1983), and again in "The Force Awakens" (2015) and "The Last Jedi" (2017) -- plus the infamous "Star Wars Holiday Special" TV movie from 1978.

Joining Ehrenreich as previously established characters are Joonas Suotamo as furry giant Chewbacca (originally Peter Mayhew until a baton-passing collaboration in "The Force Awakens") and Donald Glover as Lando Calrissian (originally Billy Dee Williams in "Empire" and "Return of the Jedi").

Those are the characters named in the Disney / Lucasfilm synopsis, though at least nine other high profile actors have been cast.

They include "Game of Thrones" and "Terminator Genisys" actress Emilia Clarke as female lead Qi'Ra, Woody Harrelson of "The Hunger Games" as Solo's mentor Tobias Beckett, plus scriptwriter Jon Kasdan (son of longrunning "Star Wars" scribe Lawrence) and assistant director Toby Hefferman ("Rogue One: A Star Wars Story") as accidentally influential pairing Tag and Bink.

Paul Bettany, Thandie Newton, and Phoebe Waller-Bridge have also been cast in undisclosed roles.

Warwick Davis, who has played everyone from Wicket the Ewok in "Return of the Jedi" to frog-like gambler Wodibin in "The Last Jedi," is also involved in an unknown capacity, as is Clint Howard, best known as the bear-befriending kid Mark from "Gentle Ben" and for roles in "Apollo 13" and "Cocoon."

After Phil Lord and Christopher Miller left the project in June 2017, Clint's older brother Ron took the reigns, having received two Oscars for "A Beautiful Mind," two Emmys for children's animated program "Curious George," and another for comedy series "Arrested Development."