What to know about 'Bridget Jones: Mad About the Boy,' as Renée Zellweger and Hugh Grant are set to return for the fourth film — but not Colin Firth

As fans will remember, "Bridget Jones's Baby" ends with Jones (Renée Zellweger) marrying Mark Darcy (Colin Firth) and raising their son, Billy.

But the new trailer for "Bridget Jones: Mad About the Boy," released on Tuesday, shows that Jones is raising two children, Billy, 9, and Mabel, 4, alone after Darcy died four years prior.

This is because the movie is based on Fielding's third "Bridget Jones" novel, where Darcy was killed by a landmine in Sudan while helping aid workers.

In both the movie trailer and book, Jones juggles raising her children, working, and facing the new world of online dating.

While Firth makes two brief appearances in the trailer, it is unclear if the film uses old footage of him or if he returned for flashback scenes.

In the third book, Jones is in her 50s and grappling with online dating, Twitter, and sexting.

There are also two new male characters: a hot 29-year-old and Mr. Wallaker, Billy's rugged PE teacher. Mr. Wallaker catches Jones in embarrassing situations, just like Darcy in the first "Bridget Jones" movie.

In 2013, Fielding told USA Today Mr. Wallaker looked like Daniel Craig but was inspired by a real-life teacher named Andrew Wallaker.

The trailer confirms that Chiwetel Ejiofor plays Mr. Wallaker, a science teacher in this adaptation, and Leo Woodall plays the 29-year-old, who appears to work in Hampstead Heath, London.

Other actors expected to appear in the film for the first time include Isla Fisher, Nico Parker, Josette Simon, and Leila Farzad.

Hugh Grant, who was absent in the third movie, "Bridget Jones's Baby," is returning as Jones' womanizing, cheating ex Daniel Cleaver. Although Cleaver was nearly killed off at the beginning of the third film in a plane crash, he survives his near-death experience.

Cleaver doesn't appear in the third book, but the filmmakers may have chosen to weave him in as a third romantic interest. In the trailer, Cleaver appears to be helping Jones look after her children and move on from her grief.

Emma Thompson reprises her role as Jones' obstetrician-gynecologist, Dr. Rawlings, from the third film, while Jim Broadbent and Gemma Jones return as her parents.

James Callis, Sarah Solemani, Sally Phillips, and Shirley Henderson return as Jones' friends Tom, Miranda, Shazzer, and Jude.