Alongside the hints that what she knows isn't the full or true story, it suggests that the transformation of Carol Danvers into a Kree warrior is going to be a crucial part of the narrative, even if it's told through flashback. Captain Marvel's official page on Twitter shared the trailer with a caption which read as "Everything begins with a hero".
The Brie Larson version of Captain Marvel is on her way via a movie set to begin its global release the week of March 8, 2019. In a voiceover, one of these noble warrior heroes (Oscar-nominated actress Annette Bening, in an as-yet unnamed role) explains to Danvers, "Your life began the day it almost ended". How good did that space fight look with Captain Marvel flying around shooting space ships? "Noble warrior heroes", Carol answers, before cracking a smile. "So you could live longer, stronger... superior".
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Having portrayed the character of Jane in 2011's Thor and 2013's Thor: The Dark World, Portman stated back in 2016 that she believed she was "done" with the MCU, although she did tease the possibility of a return for "an Avengers 7 or whatever".
Carol sees herself as a Kree as much as a human, so the fact that she leaves Earth to defend the Kree Empire in the film should come as no surprise.
The film, directed by Anna Boden and Ryan Fleck, isn't Marvel's first foray into sci-fi - that honour goes to the Guardians of the Galaxy- but it has an element of mystery that will be fun for comic book fans to unpeel.
Captain Marvel opens March 8, 2019. The audience also finds out more clues about Captain Marvel's past in the trailer.