Captain Marvel is guaranteed to cause problems for the Marvel Cinematic Universe's continuity. The movie is set in the '90s, it's inherently a retcon, and it adds yet another layer of complexity to the history of S.H.I.E.L.D. As such, the overarching continuity of Marvel's shared universe is likely to take something of a hit.

Marvel continuity has long been a tricky balance, especially during the most recent Phase 3. Told completely out of order, it's led to Spider-Man: Homecoming's "very incorrect" eight years later reference in Spider-Man: Homecoming and the question of why S.H.I.E.L.D. or the Avengers weren't called in after the incident in Missouri in Guardians of the Galaxy Vol. 2. Marvel tends to allow their directors a great deal of liberty, but as a result, when films jump around the timeline it has the potential to cause significant continuity problems.

Related: A Complete History Of The Marvel Cinematic Universe

Against this backdrop, Captain Marvel may seem smart by setting Carol Danvers' debut before any of the previous films. However, the MCU is so fleshed out that her 1990s adventures with Starforce will still have a major impact. Continuity problems don't mean Marvel films are any lesser (for the most part) but it is still an overly tangled web any new movie has to deal with. Have Anna Boden and Ryan Fleck bitten off more than they can chew?

Captain Marvel is Inherently a Retcon

Captain Marvel Young Nick Fury

Captain Marvel is the first MCU "period piece" since Captain America: The First Avenger. It's set in the '90s, and acts as a backdoor origin story to a number of key Marvel characters, most notably Samuel L. Jackson's Nick Fury and Clark Gregg's Agent Coulson. In functional terms, that means the entire concept is essentially a retcon, writing Carol Danvers into the MCU's history. And let's be clear; this wasn't planned beforehand. Marvel has a strong reputation for playing the long game, but in truth the studio is more adaptive than is typically assumed. The perfect example is Thanos himself, who was only added into The Avengers at Joss Whedon's request, and by the time they got to Avengers: Infinity War, little of the previous set up was actually relevant in the movie's outcome.

The danger with this kind of retcon is that, because it wasn't always planned into the story, it can easily contradict what's already been established. Take the character of Nick Fury; his MCU backstory has been teased throughout the movies and explored explicitly in a 2009 tie-in comic, Nick Fury: Spies Like Us. That revealed that he began his career as a member of the US military, only to be drawn into the Cold War, working behind Soviet lines on a number of occasions. Fury's performance caught the eye of Alexander Pierce, and he was invited to join S.H.I.E.L.D., becoming one of their best agents. It also shows how he lost his eye.

All of this contradicts the two-eyed, "desk jockey" version Samuel L. Jackson will be playing in Captain Marvel. Of course, these details are established in a pretty obscure source, a tie-in comic released on the Marvel.com website that most viewers have never heard of and few comic book readers will remember happened. If Marvel contradicts them, few will notice as long as the "you need to keep two eyes open" from Captain America: The Winter Soldier is honored.

Related: Every MCU Character Confirmed To Be Returning for Captain Marvel

Agent Coulson's story, however, will be known to more viewers. His introduction in 2008's Iron Man showed a competent but growing character, putting a cap on what Clark Gregg can do in Captain Marvel. More complex, his character has appeared in five seasons of Marvel's Agents of S.H.I.E.L.D. and Coulson has discussed his past with his team many times. We even know the town of his birth (Manitowoc, Wisconsin). According to Agents of S.H.I.E.L.D., Coulson studied history at college, focusing on the global significance of S.H.I.E.L.D. Swiftly recruited into the organization, Coulson attended the Communications Academy and was assigned to work under supervising officer Nick Fury. That backstory has been told in five seasons' worth of dialogue and flashbacks, although given the divide between Marvel Studios and TV, it's doubtful Captain Marvel will be bound by it.

Probably the most difficult issue of all is that, in the first season of Agents of S.H.I.E.L.D., Coulson discovered that he'd been resurrected using the blood of a Kree. A major plot point in that season was the fact that he didn't recognize the blue-skinned alien, even quizzing Sif about races with skin that color in the hope she could help identify it. Sif rattled off a dozen names; Interdites, Levians, Centaurians, and the Kree. Significantly, Coulson didn't react to the mention of the Kree, nor to Sif's mistaken belief that the Kree had never visited Earth. He'd clearly never heard of them. Continuity here is going to be very tricky indeed, especially if Coulson actually meets a Kree in Captain Marvel.

Page 2 of 2: How Captain Marvel Could Contradict the Wider MCU

Captain Marvel Already Changes Korath's Backstory Too

The wider cosmic side of Captain Marvel will also alter the wider MCU timeline, specifically Guardians of the Galaxy. In fact, whereas with Fury and Coulson things are up in the air, here we have a definite change.

The Guardians of the Galaxy Prelude revealed that Korath the Pursuer was one of Thanos's key agents, who trained alongside Nebula and Gamora. That doesn't seem to fit with Captain Marvel, which presents him as a member of the Kree Starforce who serves under Jude Law's mysterious commander. Ironically, this is the second time this particular prelude comic has been contradicted outright. The focus of the comic is on Nebula's backstory, and it's nothing like the one James Gunn revealed in Guardians of the Galaxy Vol. 2. Gunn admitted on social media that he didn't consider that particular prelude comic to be canon, and with Captain Marvel it looks as though we can discount it from continuity altogether.

Related: Captain Marvel Brings Starforce To The MCU - Here Are All 7 Members

The simple solution is that the MCU prelude comics are even less canon than many have treated them in the past. They simply don't matter. But they are built around the presentation in the movies themselves, so have a kernel of logic. Fury, Coulson and Korath's backstories may not be heavily explored on the silver screen, but the suggestion is right there to be messed with.

SHIELD/Hydra History Won't Escape Unscathed

Carol Danvers Wearing a NIN shirt in Captain Marvel

But in focusing on characters, we've ignored some bigger picture aspects of the MCU. Unfortunately, it's entirely possible that we'll see an inherent conflict between Captain Marvel and the history of S.H.I.E.L.D. as already established in the movies. According to both Captain America: The Winter Soldier and Ant-Man, in the '60s and '70s former Hydra operatives managed to infiltrate S.H.I.E.L.D. and reestablish Hydra as what Arnim Zola called "a beautiful parasite." By the end of the Cold War, they'd managed to get to the very highest levels of S.H.I.E.L.D., with Head of Defense Mitchell Carson an active Hydra agent.

Now we have another infiltration, this time by alien shapeshifters; it's already confirmed that the Skrull leader, Talos, will replace a S.H.I.E.L.D. agent. It's hard to believe that the Skrulls wouldn't unwittingly stumble upon the existence of Hydra; Hydra agents were already so ubiquitous that the odds of a Skrull accidentally replacing a Hydra operative are pretty high. The Skrulls' usual modus operandi is to use their shapeshifting powers to take advantage of pre-existing divisions and weaken a planet's defense; the hidden split between S.H.I.E.L.D. and Hydra would provide them with a tailor-made opportunity. Marvel will need to tread very carefully indeed in this regard, as it would be very easy to unwittingly cause a continuity problem here.

-

None of these problems are insoluble. The issue with the tie-in comics is resolved by simply choosing a model of canon in which the movies themselves take precedence; meanwhile, careful writing can avoid continuity issues between Captain Marvel and the larger MCU. Still, by treading on an area already so widely covered with so much new material, it's almost certain that there are going to be some significant issues with the timeline after Captain Marvel's release. Hopefully they won't be on the scale of Spider-Man: Homecoming's "eight months later," and will be easy to explain.

More: A Complete History Of The Marvel Cinematic Universe

Key Release Dates