Despite her enormous power, Captain Marvel isn't the perceived, all-powerful weapon for the majority of her own movie.
**SPOILER WARNING: This story contains spoilers for Captain Marvel.**
For the longest time, we thought the Tesseract, the glowing cube container for the Space Stone, had been hanging out in some S.H.I.E.L.D.-sanctioned facility. Maybe it was brought out on occasion for various tests, but, otherwise, we thought it lived a quiet, hero-less life post-Captain America: The First Avenger until popping onto Loki's radar in 2012's The Avengers.
Captain Marvel freed us of this assumption.
The Tesseract's involvement in Captain Marvel was a surprise, to say the least, but it probably shouldn't have been. That damn cube was responsible for a lot of bad in the MCU's Phase One; Red Skull and the rise of Hydra for one, and the Battle of New York for another. Given that Captain Marvel takes place between (most of) Captain America and The Avengers, it was prime real estate for a throwback to simpler times — a time before the Space Stone helped take out half of all life in the universe.
But amongst all that bad Space Stone press, between the Nazi weapons and opening portals for alien armies, the Tesseract actually did some good in the universe by making Captain Marvel. All-consuming power, it turns out, can be positive. It just depends on who wields it.
The Tesseract's involvement in this film, though, raised a lot of questions amongst fans. How did it get there? What's it been up to? Hasn't this thing been around for long enough? The answer to all these questions (and more) can be found below in this brief history of the Tesseract's time in the Marvel Cinematic Universe.
FROM ASGARD TO EARTH
The First Avenger introduced Johann Schmidt (the soon-to-be Red Skull) in its opening scene as he monologued about the Tesseract's glorious history. "The Tesseract was the jewel of Odin's treasure room," he says before finding and taking it. Odin brought the Tesseract to Earth for safekeeping, and, for centuries, humans guarded it as they worshipped the Asgardian gods.
Through his research, Johann learned of the Tesseract's powers and sought it out for Hydra's use. As we all know, this would go horribly wrong, Red Skull would eat it (and end up on another planet as the cursed guardian of the Soul Stone), and the cube would go down in the Arctic alongside Captain America.
The old man keeping guard over the Tesseract in a church in Tønsberg, Norway, said it best: "You cannot control the power you hold. You will burn!"
HOWARD STARK AND S.H.I.E.L.D.
As far as we knew 'til Captain Marvel, this is where the Tesseract's story stopped until The Avengers.
At the end of Captain America, Howard Stark and a team of technicians find the Tesseract at the bottom of the ocean. It seems they'd traced its energy signature in the hopes of finding Cap, something they didn't end up achieving for decades. But they did the Tesseract — and boy, did it cause some more problems.
MAR-VELL AND THE KREE-SKRULL WAR
Mar-Vell, a Kree scientist who goes by the name Wendy Lawson, comes to Earth in order to work on a Light-Speed Engine that will help the Skrull escape the Kree's grasp. We can assume she came to Earth specifically because she knew the Tesseract — and, therefore, the Space Stone — was kept here, but we're still unsure how the Tesseract came to be involved with her project after Stark found it.
Enter Carol Danvers, who helped Dr. Lawson test this Light-Speed Engine as part of a joint U.S. Air Force/NASA program called Project P.E.G.A.S.U.S. Dr. Lawson harnessed the Tesseract's energy, made the Light-Speed Engine, and was then foiled by Yon-Rogg and the Kree. Yon-Rogg killed Lawson and, to keep the Engine out of Yon-Rogg's hands, Carol destroyed it.
Carol then accidentally absorbs the Tesseract energy contained within the Engine, which gave her all those fantastic cosmic powers.
But the Tesseract itself wasn't in the Engine, just some of its energy. The Tesseract was actually sitting up in Lawson's Earth-orbiting laboratory and stayed there until Carol returned to Earth six years later and regained her memories. Carol, Nick Fury, Maria Rambeau, the Skrull Talos, and Goose the cat/Flerken find the Tesseract, put it in a Fonz lunchbox, and save the day before relinquishing the Tesseract to S.H.I.E.L.D. for safekeeping.
THE BATTLE OF NEW YORK
Assuming the MCU doesn't pull another fast one on us, we can safely say the Tesseract chilled out in a S.H.I.E.L.D. storage/testing facility for the next couple of decades. Fast-forward to 2012, and Loki arrives on Earth, steals the Tesseract out from under S.H.I.E.L.D.'s collective nose, and kidnaps Dr. Erik Selvig to build a portal using the Tesseract's Space Stone powers.
They succeed, and the portal opens up above Manhattan to unleash an army that would raise the MCU's stakes forever.
Side note: Of course, Loki isn't the only one using the Tesseract for less than savory purposes, as the Avengers discover that S.H.I.E.L.D., under orders from the World Security Council, has been developing Hydra-esque weapons using the Tesseract's considerable energy. Not cool, guys. You just know that throughout The Avengers, Nick Fury was thinking "Not this motherf***ing thing again."
The Avengers eventually defeat Loki, though, and Thor takes his misbehaving brother and the Tesseract back to Asgard.
FROM EARTH TO ASGARD
With the Tesseract on Asgard, Heimdall uses its energy to repair the Bifrost before returning it to Odin's Vault alongside a host of other magical items. Thor: Ragnarok happens (watch the movie if you want to know anything beyond that 'cause I ain't explaining it here) and when Loki goes to destroy Surtur's crown, he comes across the Tesseract.
By the time Ragnarok is over, we're unsure if Loki actually did take the Tesseract, but, like, we knew. This is Loki we're talking about. Of course he took the stupid Tesseract.
"YOU REALLY ARE THE WORST BROTHER"
Enter Thanos and his genocide scheme. The Asgardians' ship is attacked, people die, and Loki gives up the Tesseract to save Thor. Thanos crushes the Tesseract in his fist like it hasn't been a plot device for 10 years or whatever to retrieve the Space Stone.
Then half of everyone dies. The end.