Serenity is an amazing sci-fi film that takes place after the events of the final episode of Firefly, and was the first project he directed (film or TV) since the show, three years earlier. After the cancellation of Firefly, and several failed attempts to shop the series to different networks, Whedon decided to turn his vision into a feature film to give the fans some closure.
It follows the crew of the Serenity as they continue to run from the Alliance, while harboring the brother/sister fugitives, Simon and River Tam. This has made getting jobs much more difficult and now they are beginning to get desperate. When they take River on a job during which Reavers attack, Simon decides that its time to leave the crew of the Serenity for his and River's safety.
Then, as River is watching a Fruity Oaty Bar commercial (which was inspired by Mr. Sparkle, the mascot of a fictional brand of dish-washing detergent, who was featured in The Simpsons episode "In Marge We Trust"), she mutters a single word, "Miranda", and goes berserk, nearly shooting Capt. Reynolds.
Now, an Alliance agent known only as "The Operative" (played by Academy Award nominee Chiwetel Ejiofor) is on their tracks and Mal and the rest of the crew are determined to find out what "Miranda" is and what the Alliance is hiding.
Even though this film didn't do as well as hoped in the theaters ($38.9 million worldwide on a $39 million production budget), that doesn't take anything away from the quality of the film or the desire of the fans to see more from this world. It's such a rich universe, filled with possibilities. It's even spawned a series of comic books, continuing the story of these beloved characters.
Whedon had written or co-written (as a script doctor) several films prior to this, including Buffy: The Vampire Slayer (the movie), Speed, Toy Story, Titan A.E., and X-Men, but had only directed television series. This opened up a whole new world for him.
Due to his cult status in the geek world, his great work on several different comic book series, and his amazing directing talent, especially in Serenity, Whedon was given a once in a lifetime opportunity to direct what would become the greatest superhero film to date...The Avengers.
He has now had his hands in many of Marvel's recent projects, such as Thor, Captain America: The First Avenger, Thor: The Dark World, Agents of S.H.I.E.L.D., and the much anticipated Avengers: Age of Ultron. He also recently wrote and directed his own film, Much Ado About Nothing.
With such a stellar career, it's hard to imagine that he is just now beginning to hit his peak. Whedon has quite a future to look forward to, and I personally am excited to see where it goes.