Spoilers for Avengers: Infinity War follow below.
We’ve known for some time that the plot of Avengers: Infinity War would follow Thanos (Josh Brolin) attempting to collect all six Infinity Stones, which would then give him the power to rule the universe—or to wipe out half its population with the snap of his fingers. Indeed that’s the main “plot” of Infinity War, but the film also digs a little deeper into the Infinity Stones themselves and what they do. There are six in total: The Space Stone, the Mind Stone, the Reality Stone, the Time Stone, the Power Stone, and the Soul Stone.
It’s this last stone that proves the most impactful in Infinity War, as Thanos’ journey to find the Soul Stone takes him to Vormir, the planet where the Soul Stone is hidden. Thanos brings his daughter Gamora (Zoe Saldana) with him, and they encounter Red Skull who serves as the keeper of the Soul Stone. Red Skull first appeared in Captain America: The First Avenger as played by Hugo Weaving, but at the end of that movie the Tesseract (which is powered by the Space Stone) transported Red Skull to parts unknown—now revealed to be Vormir. In Infinity War, Red Skull is played by Ross Marquand and informs Thanos and Gamora that in order to retrieve the Soul Stone, one must sacrifice the soul of someone they love. So Thanos, tears in his eyes, promptly throws Gamora off the cliff.
That’s a lot of screentime devoted to a single Infinity Stone, and indeed the Soul Stone may hold the key to what happens at the end of Infinity War. If you’ll recall, once Thanos gets all six stones, he snaps his fingers and half the population of the universe disappears into dust—including half of the Avengers.
In the comics, the Soul Gem (they’re called gems in the comics, not stones) allows the user to steal, control, and alter souls both living and dead. The Soul Gem is also, in the comics, home to a “pocket universe,” in which souls can be trapped. As alluded to in Infinity War, the Soul Stone has been known to show evidence of sentience, even acting independently. When the Soul Stone collects someone’s soul, it stores that soul in the stone itself—either in a pocket universe or some strange world.
Is this where all the people who turned into dust went at the end of Infinity War? It’s certainly possible, and would explain why so much screentime was devoted to this particular Infinity Stone. This could also set up a visually dynamic Avengers 4, as the user of the Soul Stone has the ability to go inside the Soul World and visit those trapped inside the stone.
This information, of course, comes from the comics, and as we’ve seen in the Marvel Cinematic Universe the comics only serve as a foundational basis for a lot of what happens. So we can’t exactly say for sure that the Soul Stone in the MCU will have the same abilities and effects as the Soul Gem from the comics, but it does seem to make sense that this would be one of the major keys to Avengers 4, which will conclude the storyline that began in Infinity War.
Of course this also means that those characters who died of means other than the Thanos Snap may indeed be lost forever, unless someone can manipulate the Time Stone enough to change the events that left Gamora, Loki, and others truly dead.