Avengers: Infinity War is less than two weeks away, and with it should (hopefully) come some answers that we've been asking since the release of 2012's The Avengers. The most pressing question has been the location of all the Infinity Stones, and seven years after the very first one made it's debut in Thor's post-credits scene we still don't know where they all are. Or rather we don't know where the soul stone is. It's the last one to find, and people have been losing their minds trying to figure it out. Figuratively speaking.
There have been a lot of theories as to where the Soul Stone is, some of them more sensible than others. A popular theory is that Heimdall has the soul stone, which is what gives him his unique ability to watch over the nine realms. Others theorised that it might show up in Captain Marvel (not outside the realms of possibility), that it's in Wakanda (already debunked), or that it's lost in time and Thanos needs the time stone to retrieve it. Others are downright daft, like claims that Tony Stark is the soul stone, thanks to a promo poster that put the two together, or that his arc reactor is somehow an ancient cosmic artefact - despite the fact we've literally seen him building them.
But I have an alternate theory based on what we've seen so far. I think the explanation is a lot simpler and that the soul stone is actually...
Turn away if you hate potential spoilers, seriously.
My theory is that the soul stone is actually on Thanos's home world, Titan, and Thanos knows this.
It's worth mentioning before I get started that I'm basing most of this on a lot of assumptions. There's a lot we don't know about the film considering it won't be out for another two and a half weeks, and I'm trying to put a lot of pieces together based on information from trailers and other marketing materials. The first is the assumption that Thanos collects the Infinity Stones in a particular order. All the promo artwork suggests that he gets the purple power stone first, followed by the blue space stone second, the red reality stone third, and then the orange soul stone fourth.
We've known that Thanos gets the first two in that order based on the trailers, which actually show him adding the space stone to the Infinity Gauntlet with the power stone already in place. The order he gets the next two in was always an assumption, but was recently corroborated by a promo trailer posted to Marvel's Twitter account. Watch closely at 0:24 and you'll see Thanos emerging from a wormhole. Look even closer and you'll see that he actually has the first four stones already - in the order originally laid out by promo art.
— Marvel Entertainment (@Marvel) April 7, 2018
Here's a screenshot in case you don't see it.
Infinity stones are uniquely coloured, and while it's not totally clear it's definitely orange. It's certainly not green or yellow, which means it can't be the time stone or the mind stone. The key to this theory is what we've seen in the trailers. We know from the last full trailer that Thanos's Q ship leaves New York at some point and ends up on an alien planet - a planet that since been confirmed to be Thanos's home world. While some had speculated that this planet's orange hue suggested it was inside the soul stone, seeing as how the comic version does house an entire world populated by the souls of people who had been absorbed by the gem, or possibly the stone itself, but I don't think this is likely.
The question is why Thanos's ship went to Titan in the first place. We know that Spider-Man, Iron Man, and Doctor Strange wind up on Titan, likely on board said ship, where they hook up with the Guardians of the Galaxy. We similarly have to ask why the Guardians are there in the first place, telling Tony Stark that his plans suck.
We also have to consider the fact that Titan is desolate and seemingly devoid of life, and that Thanos has some sort of warped reasoning for completing the Infinity Gauntlet. Marvel Studios head Kevin Feige has confirmed that those two things relate to each other, telling Entertainment Weekly:
“He’s from a planet called Titan that’s no longer inhabited because of things that he thought he could help prevent, and he was not allowed to do that.
What he feared most happened, and the planet and everybody on it basically went extinct. He vowed not to let that happen again. He thinks he sees the universe going down the tubes. He thinks he sees life expanding outward unchecked. That will bring ruin, he believes, to the universe and to that life.”
We don't know how that happens right now, but what could possibly cause an entire planet to die? Well we've already seen that in Guardians of the Galaxy when The Collector shows the Guardians footage of a celestial using the Power Stone to wipe out an entire planet in the blink of an eye. We don't know what became of that planet, but it looks like it was ravaged by the power of the stone. That's a lot of power, and according to the Infinity War's tie-in comic prequel the soul stone is actually the most dangerous stone of them all.
The key thing about Titan, as we've seen it in the trailers, is that it's devoid of life but the architecture still seems intact. Heavily decayed, but still in existence. The Power Stone seemingly destroyed everything in its path, but the soul stone? Given its comic abilities there's a strong possibility that if mishandled it's capable of wiping out life on that same sort of scale. But the key is killing the life, while leaving the infrastructure intact. If someone has mishandled the soul stone and wiped out life on his home planet, that could spur Thanos into collecting the stones into his own more-capable hands and prevent such a travesty from happening again.
If the stone destroyed life on Titan there's the chance it's still there, and as we've seen it's not uncommon for Infinity Stones to be left lying around on empty planets for just about any-old a-hole to find. But if the stone is as dangerous as everyone says,he could have left it there until he was able to safely control the stone's power. Plus if he was the only survivor, and thus the only one who knew of the soul stone's involvement in Titan's destruction, he could leave it there safe in the knowledge that it will stay hidden.
Now the question is, why didn't Thanos pick up the soul stone first? We've seen he gets the power stone first, which is under heavy guard on Xandar. It's possible that it's all down to the fact that he wanted to do things on the down-low, or low-key if you're in the pun business. That's why he used initially used intermediaries rather than collecting the stones himself. Loki was dispatched to get the space stone from Earth (with the mind stone, which he promptly lost), and Ronan was tasked with collecting the power stone from Morag. If the soul stone is on Titan, then Thanos knew where it was and knew exactly what sort of defences it had, so there was no rush to find it and claim it. He had the soul stone on Titan, the gauntlet locked away in a vault, and the sceptre housing the mind stone in a third place before it was given to Loki.
Plus, considering the fact that the reality stone was lost between universes when his quest began, there was no telling when he'd be able to claim them all. Keeping them separate meant they were kept somewhere safe until the time was right to unite them. As Volstagg says at the end of Thor: The Dark World, it's not wise to keep more than one Infinity Stone in the same place.
Extra spoiler warning: there are confirmed spoilers beyond this point
So why fourth? Why not last if that's all the case? I have an idea about that too. As we know, the Guardians end up on Titan for part of the film, where they team up with Tony, Doctor Strange, and Spider-Man. We also know that the Q ship is over New York before promptly blasting off and eventually crash landing on Titan. Why would it suddenly leave like that? And how are the Guardians there by pure coincidence? My guess is that it's a trap.
In accordance with what we've seen, we know that Thanos gets the red reality stone third. We also know that it was handed over to The Collector on Knowhere by Volstagg and Sif, and thanks to leaked footage from last year's San Diego Comic Con we know that the Guardians arrive on Knowhere at the same time as Thanos. We don't see the scene in full, but it's more than possible that Thanos gets distracted by Drax trying to kill him while the Guardians pinch the stone for themselves.
As for their ability to get there at the perfect time, it's worth remembering that they find Thor near the start of the film. Thor collected the Aether from Malekith, and likely knew about Volstagg and Sif's mission to take it to The Collector. If he didn't know at the time, he'll have been told by Loki who was masquerading as Odin at the time and likely gave the order. It's a 10 second conversation, Thor mentions he was looking for them, Loki reveals what he knows, then Thor eventually passes that information onto the Guardians. Thanos himself likely got the information in a similar way, by interrogating Loki after retrieving the space stone.
Past trailers have shown Thanos in possession of two Infinity Stones during the battle on Titan, and assuming Marvel haven't deliberately missed out other stones to help us avoid spoilers my guess is that the Guardians take the reality stone to Titan to draw Thanos into a trap. On his home planet because they have a flair for the dramatic. Once he figures that out, the Q ship blasts off the provide Thanos some support on his home planet, accidentally bringing three Avengers along for the ride. There they hook up with the Guardians and that's how the two teams come together.
Obviously things don't work out so well whatever happens, since we've seen Thanos has the reality stone in the trailer, and while he's on Titan he might as well pick up the soul stone before going after the other two. That shot up top doesn't make it clear whether he has the time stone (the thumb is obscured), but it seems unlikely that he would be in such close proximity to Strange's Eye of Agamotto and fail to realise its significance. After all we've already seen Ebony Maw will be torturing Doctor Strange for one reason or another. It's possible that this sequence ends with Strange being taken captive, and Maw trying to pry information about the stone away from him - possibly information about how to safely remove the stone from its housing.
I may be completely wrong here, I'm more than willing to admit that there's a lot we don't know that could completely tarnish the conclusions I've put together. For the time being, based on everything we've seen in the trailers and promotional materials, the idea that the soul stone is on Titan - or at the very least has a history with the planet - makes the most sense to me. We're just going to have to wait two weeks to find out for sure.