Whether you prefer to take it in the spirit of sincerity or deadpan self-satire, the Fast & Furious franchise has for awhile now gotten a lot of mileage out of winking at the notion that the enduring car-stunt series is actually grounded in an earnest message about the importance of (as Dominic Toretto pronounces it) "Fambly." The post-biological band-of-brothers is framed as a group so tight-knit that it scarcely matters that no one can really pinpoint where they transitioned from drag-racing freeway-pirates to a sought-after anti-terrorism mercenary squad.
Obviously, fans were expecting something drastic to change about the franchise foundations for this kickoff to the purported trilogy of features set to bring the series to a definitive endpoint in Part 10. By now the series has done just about as much as can be done with the "series all-stars do a job" premise that's been fueling the brand since Fast Five; and with onetime second-in-command Paul Walker having passed away tragically during the production of Furious 7, it was clear a new direction would be necessary.
But nobody could've predicted a story shift as out-of-nowhere as what was revealed in the Fate of The Furious trailer; which opens with what initially looks like a mystery ("Toretto has gone rogue - why?") only to quickly reveal the big hook: Dom has fallen under the spell of Charlize Theron's "Cipher," an international criminal mastermind who has turned him against his former teammates as part of a sinister global terrorism scheme. Not only is he working for a villain, they also appear to be an item - which comes as a shock to Michelle Rodriguez as Toretto's now cured-of-amnesia wife Letty.
So what's going on here? Is Toretto fully evil? Are we being tricked? Is this all some sort of elaborate setup for a third-act reveal? It's difficult to tell - especially considering the bizarre place that the Fast & Furious movies occupy in terms of the blockbuster scene ever since the franchise went "post-modern" somewhere between the fourth and fifth installments.
At this point, these are essentially superhero movies, albeit ones set in a bizarre alternate universe where both superhuman fighting abilities and high-level government clearance appear to be conferred on individuals in direct proportion to their skills at driving ostentatiously-outfitted gas-guzzlers very, very fast. But despite having leapt into a strange genre-limbo between straight-faced macho drama and absurdist self-satire, the franchise has yet to fully make the leap into outright science-fiction and/or fantasy like the James Bond movies eventually did with Moonraker (or, some would argue, earlier with You Only Live Twice and Live And Let Die.)
Oh sure, it's come close: Parts 6 and 7 both had plots built around high-tech macguffins that couldn't possibly exist in reality, while 7 additionally featured a climactic showdown with a weaponized drone aircraft that effectively took the series right up to the line of having The Family Toretto go up against a killer robot - and, let's face it: Would it really surprise anybody if at some point between now and Part 10 the heroes are pitted against a squad of evil, artificially-intelligent self-driving cars?
Even still, that's not quite a full break with reality (especially considering Google in real-life is dumping millions into perfecting said driverless cars) and that matters because it means, absent any other evidence, that it's likely Dom's heel-turn will have a semi-logical explanation. If these were The Avengers, for example, you'd basically have to throw up your hands and wait since a Marvel superhero can turn evil for almost unlimited reasons (alien abduction, clone-replacement, love-potion, Infinity Stone, deal with Literally The Devil, deal with any of a dozen demonic entities merely "approximating" The Devil, any of a thousand different forms of mind control both science and magic based, etc); but this is Fast & Furious - as of yet, we don't know of any aliens or wizards stalking about.
That means that unless Toretto is only "pretending" to go bad (which, honestly, is probably the most likely scenario), his newfound treachery is likely grounded in a rationale that makes sense within the series' established reality. And since (unless you forgot) pretty much every character except Walker's, The Rock's and Elsa Pataky's technically started off as cold-blooded criminals, it's not too outrageous to assume that old habits would die hard.
On the other hand, Toretto was always previously framed as a reluctant baddie who genuinely cared more about his family unit, so the lure here would have to be either pretty extreme or come with an additional incentive. Maybe Cipher is capable of killing off his whole team unless he helps. Maybe she's threatened the safety of (in canon) retired Brian O'Connor and his wife... who also happens to be Dom's sister. Maybe she has a previously unknown connection to his past - we don't actually know much about the Toretto family history, save that he was banned for life from racing professionally for assaulting the driver who accidentally killed his father. Maybe this is all somehow connected to Kenny Linder, the victim of Dom's near-fatal wrench-beating and about the only minor piece of Furious backstory to not yet be repurposed for later-series supervillainy.
There's also an open question of what's driving the other big shakeup: Jason Statham's Deckard Shaw - the villain from Furious 7 and brother of the currently-comatose villain from the sixth film - has been released from prison in order to help the team take down their now-former leader. Obviously, the base logic of why he'd be a helpful teammate is evident in that he happens to be a one-man army proficient in nearly all forms of weaponry, hand-to-hand combat and gifted with superhuman endurance (in other words, a character portrayed by Jason Statham) - but is there a plot specific reason that he's needed? It wouldn't be outside the rhythm of the franchise for the Shaw Brothers to have history with Cipher, up to and including her possibly being a Shaw herself.
Speaking of the Shaws: It's also widely believed - though not confirmed - that Helen Mirren's as-yet unannounced mystery role may end up being the matriarch of the Shaw family. If so, a "family versus fambly" storyline would be all the more reason to find out Cipher is connected in some way... but where would that leave Toretto? Really, it's anybody's guess.
Or is it? As mentioned previously, outright mind-control doesn't yet exist elsewhere in the franchise, but "issues" involving memory and loyalty absolutely do. After being believed killed-off prior to the events of Part 4, Rodriguez's Letty returned in Part 6 with the explanation that the attempt on her life didn't kill her - it merely left with incredibly specific amnesia that erased all knowledge of her identity and past but preserved her fighting skills, encyclopedia automotive knowledge and criminal aptitude.
Even among Fast & Furious fans, Letty's bizarre amnesia (which led to her working for Shaw's crew in Part 6) is considered a somewhat extreme plot-hole, and theories (or, rather, hopes) for a more detailed explanation have been bandied about for awhile now. If Cipher does turn out to be part of the Shaws' business (by relation or otherwise), manipulating Toretto with some form of high-tech mind-manipulation (hypnosis, even?) could be handily "explained" by revealing that the same method was used to mind-wipe and control Letty.
It's likely that fans won't really know what's happening to their favorite terrorist-hunting tank-top enthusiast until The Fate of The Furious opens on April 14th, 2017 - which is exactly how Universal Pictures wants it.