When we left superspy Jason Bourne at the end of The Bourne Ultimatum he had dived off the top of the CIA's New York headquarters and into the East River after discovering the shocking truth about his identity - that he'd volunteered to become an assassin for the black ops program codenamed Treadstone.
Initially, we think that Matt Damon's floating Jason died in the fall. We then jump forward to a TV report saying that no body was found, then to Julia Stiles' rogue operative Nicky Parsons smiling knowingly and then back in time to Jason, who kicks his feet and swims to freedom. The next Bourne sequel locked and loaded.
Or was it? The Bourne franchise is one of Universal Pictures' most lucrative and it clearly went to some lengths to ensure that Jason Bourne was alive and ready for further adventures.
However, its two main men, star Matt Damon and director Paul Greengrass, had grown weary of the character and were anxious to move on with their careers. Indeed, Damon was adamant the series had run its natural course.
Over three critically acclaimed full-throttle episodes, in which founding director Doug Liman and British docu-drama specialist Greengrass had reinvigorated the language of the action movie, the amnesiac assassin Bourne traced his crimes back to the highest echelons of the US government and liberated himself.
The third member of the Bourne brains trust, screenwriter Tony Gilroy, also baulked at doing another Bourne movie. He was now a hot Hollywood property after writing and directing Michael Clayton, which earned seven Oscar nominations, and didn't need to grapple with the problem of another sequel.
Another obstacle to bringing together Damon, Greengrass and Gilroy for a new Bourne flick is that the three don't see eye-to-eye. The Bourne series had been plagued by acrimony and division since the first instalment, the result of deep temperamental and creative differences between the key creatives.
The dispute went public in 2009, when Gilroy complained bitterly in an article in The New Yorker about how Greengrass had directed his script for The Bourne Supremacy, saying his failure to make Bourne atone for his violent crimes was "a crime against the gods of storytelling".
Damon, who is unfalteringly loyal to Greengrass, responded by telling GQ magazine that Gilroy's script for The Bourne Ultimatum was "unreadable . . . I could put this thing up on eBay and it would be game over for that dude." Damon also said he would never make a Bourne movie without Greengrass and vice versa.
So Universal was stuck with the problem of how to make a Bourne movie without Jason Bourne.
They could have simply slipped another actor into the role of Bourne in the same way that the James Bond franchise had been doing with reasonable success over the decades (and with real success in the case of Daniel Craig).
But that would be an affront to the realist aesthetic of the Bourne series.
While the Bond movies have always had a slightly campy, winking-at-the-camera quality, the Bourne films thrilled because they felt so anchored to our world (indeed, when the Bond franchise was rebooted with Craig, the Bourne influence was clear).
"My argument was, 'No one would believe it'," said Bourne producer Frank Marshall of the idea of recasting Jason Bourne. "I would have laid down in the street to stop that."
When Gilroy came back into the fold, his solution was not to move forward in time (the traditional sequel) nor backward (the prequel) but leap sideways - to follow a parallel narrative to the one we've watched over three movies, in which Damon's Bourne battles to solve his identity while keeping one step ahead of the people who created him.
In expanding the Bourne universe, Gilroy imagined an even grander and more sinister clandestine operation called Outcome in which the US government is using drugs to alter the chemistry of soldiers, including Jeremy Renner's Aaron Cross, to improve their muscle efficiency, neural regeneration and pain suppression.
When the government decides to wipe out the program, Cross recruits a research scientist (Rachel Weisz) to outfox the secret agency headed by Eric Boyer (Edward Norton Jr) that runs the Department of Defence experiment.
The film includes several brief scenes with veterans of the previous Bourne films, including Joan Allen, Albert Finney and David Strathairn.
While moving sideways from the Bourne narrative feels like a clever commercial solution to the problem of making a Bourne movie without Bourne, it is very much in keeping with the kind of mind-bogglingly complex narratives typical of Gilroy, who kept audiences on their toes with Michael Clayton and Duplicity.
He believes that audiences are now so sophisticated after watching movies like Memento and Eternal Sunshine of the Spotless Mind that filmmakers have to try new ways of challenging them and keeping them guessing.
Indeed, far from a drawback, the absence of Jason Bourne has given Gilroy a chance to explore his ideas about sophisticated storytelling, something he says is inevitable in this age of mega-movies, sequels and spin-offs
Most fascinating, Gilroy has The Bourne Ultimatum running in the background of the first 15 minutes of The Bourne Legacy, something he believes has never been done before (although it is commonplace in television).
"As we enter this new age of dynastic filmmaking - with these gigantic films - everything has become sort of episodic in an interesting way," Gilroy told Film Ink.
"There are these new storytelling techniques and what we've done here is an interesting way to do it, but I don't think it will be the last time it is done."
Despite the acrimony that has spawned a Bourne movie without Jason Bourne, producer Marshall still has hopes that a fifth movie will involve both Damon and Jeremy Renner. It something Gilroy would like to see, he told The Huffington Post, but he's not all that confident it will happen.
"I wanted to make sure, from the very first day, that Jason Bourne was in this world and that Matt Damon was never replaced.
"And I don't think there was anybody more interested in serving the quality and integrity of what happened before.
"What goes after that? I have no clue."