The Soul Stone was the biggest mystery going into Avengers: Infinity War, and while what Marvel had in store the last known Infinity Stone was definitely not what was expected, it was exactly what was needed.
The basic premise of Infinity War was that it was going to bring together all the disparate threads of the MCU, chiefly the Infinity Stones which had been MacGuffins across the previous eighteen movies: the Space Stone in Captain America: The First Avenger and The Avengers (with a return in Thor: Ragnarok); the Reality Stone in Thor: The Dark World; the Power Stone in Guardians of the Galaxy; the Mind Stone in Avengers: Age of Ultron (after its introduction in The Avengers); and the Time Stone in Doctor Strange. After that last one, all eyes were on the ethereal Soul Stone, with it expected in Guardians of the Galaxy Vol. 2, Thor: Ragnarok or Black Panther.
Except it wasn't anywhere to be found. Despite Marvel's modus operandi of seeding all the Infinity Stones throughout the MCU before, they held the Soul Stone back, leaving it entirely for the plot of Avengers: Infinity War. Fans were confused, they were worried - but they needn't have been. The eventual real was perfect.
Soul Stone Theories Set Us Up For Disappointment
By the time Avengers: Infinity War became known a tangible endgame (the two-parter was announced in 2014) and audiences were getting up to speed with the Infinity Stones (Thor's subplot in Avengers: Age of Ultron was pretty overt), we already knew the location of four of the Infinity Stones. This put extra onus on the ones to come, and when Doctor Strange expectedly revealed the Time Stone, everything weighed on the Soul Stone.
Theories predate it being the lone unknown, the most popular being that it was contained inside the all-seeing Heimdall (something that could have been the plan before Thor: Ragnarok pivoted away from Age of Ultron's setup), then that it was the source of Wakanda's Heart-Shaped Herb. As time became shorter, though, things got crazy: it's inside Odin and taken by Hela (another Ragnarok change possibility); the Soul World is Titan; it's lost in time; it's linked to absentee Adam Warlock; Thanos already has it and this entire discussion is misdirection. The trailer shot of Captain America taking on Thanos even has some speculating Steve Rogers was the Soul Stone.
Read More: The Craziest Soul Stone Theories
That last one aside, each theory had enough grounding to be captivating, and that was where the problem came. As exemplified far-too-perfectly by Star Wars: The Last Jedi, fan theories can go more intricate and sprawling than even the most ambitious mainstream movie franchise, and so there was the prime opportunity for disappointment: either the Soul Stone would be as expected and thus predictable, or it would struggle to live up to the bizarreness made up by fans. Amazingly, it defied both fears.
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- The Avengers 4 / Avengers: Endgame (2019) release date: Apr 26, 2019