O Soul Stone, Soul Stone, wherefore art thou Soul Stone? Not with Adam Warlock, it seems. As we draw closer to the release date of Avengers: Infinity War, theories about the location of the final Infinity Stone, the Soul Stone, have been piling up. Of the many theories out there, a popular one has been that the stone is with its original owner from the comics, Adam Warlock. The character was teased in one of the after credits scenes in Guardians of the Galaxy Vol. 2, but it sounds like fans shouldn't anticipate Adam making his debut in Infinity War, according to James Gunn.
One confident fan on Twitter shared his thoughts on the whereabouts of the Soul Stone, writing that he was 99% sure that it was with Adam Warlock. James Gunn, director of the Guardians of the Galaxy movies, replied to the post and wrote back "You would be 99% incorrect then." Gunn is flat out saying that Adam Warlock does not have the Soul Stone, and that the Avengers will have to go somewhere else to find the final piece of the Infinity Gauntlet.
In the comics, Adam Warlock has a big role in the Infinity Gauntlet storyline, which seems to be a partial inspiration for Infinity War. Adam has the Soul stone as a gem in his forehead, which Thanos eventually takes from him, leading Adam to recruit Earth's heroes in a mission to take the gauntlet from the Mad Titan. This has led to theories that the movie may follow a similar route, but there doesn't seem to be a particularly strong case for it.
There's been no indication that Adam is in the movie, though, given how secretive the marketing has been, it's possible that he's being kept under wraps to prevent spoiling too much. James Gunn, who rarely lies about this kind of thing, has previously said we shouldn't assume that Adam is in Guardians of the Galaxy Vol. 3, so there's already a lot of mystery around this character. However, it should be noted that at least part of Adam's story has already been given to the Vision, who has the Mind Stone in his head.
As for the other possible locations of the Soul Stone, perhaps the most popular theory is that it's on Wakanda, which is what brings Thanos' army knocking on the nation's doors. However, Ryan Coogler has said that Black Panther does not tease the stone in any way. Another theory (and in my opinion the strongest one) is that the Soul Stone is on Titan, Thanos' homeworld. It would certainly explain what brings Iron Man and his team all the way to outer space and it's perhaps no coincidence that this world is orange, the color of the Soul Stone.