So with many datamines showing more of Side Order's possible enemies (no model shown yet, just the data), I think it's safe to speculate on the possible story. Now here are the most obvious confirmations out of the way first:
The Octoling we will be playing is definitely not the same as the playable Octoling we already have available for Splatoon 3, as the one seen in Side Order looks noticeably older. Pearl and Marina will be involved in the story in some way, their roles however, are unknown at the moment, but they will play some sort of important role in the story. Whether this is just in that Octoling's mind (I doubt it due to the presence of other characters' role in the story) or Inkopolis Square really did lose all color remains to be seen.
Now for the not-so-obvious details (with some speculations thrown in to make sense):
The official Splatoon Twitter said something like "getting to the bottom... or top of this". This is a possible hint as to what will be involved in the story, this brings me back to the idea that Agent 8 (or not-Agent 8) will be descending that elevator in the images to bring the Octarians to the surface, thus calling back to the tweet adding "or top of this."
Another detail that's not-so-obvious is the noticeable absence of DJ Octavio in the Return of the Mammalians story, where he only appeared at the beginning (and optionally through a memory) and at the climax of the story to help Neo Agent 3 defeat Mr. Grizz.
This makes me think that he will have a very important role in the story of side order, perhaps the same role as Craig Cuttlefish once had in Octo Valley and Octo Expansion. So this could mean that Side Order could take place at the same time as Return of the Mammalians, simply because by extension, Pearl and Marina would also have a good reason why they weren't involved either.
So with all of the details (and some speculation) out of the way, here's what I think the story of Side Order will be:
Agent 8 (or not-Agent 8) will first appear in a customization screen, with the text reading something along the lines of "Who am I?", the text itself will not change after you picked the character's gender, it will stay there as you choose further customization options, such as skin tone and eye color. After the customization has been decided, Agent 8 (or not-Agent 8) will wake up in Inkopolis Square, unsure of what happened that caused the place to be drained of color. He/she will then get a call from Pearl, which will be static-like at first, but will clear up as Pearl talks further about Marina's disappearance. After Pearl suggests that Agent 8 (or not-Agent 8) to first investigate the mysterious building that seems to have replaced the Deca Tower, the Octoling goes inside, which is revealed to be an elevator.
For now, none of the floors are accessible, with the investigation at seemingly at a dead end, Pearl doesn't know what to do, until a voice that sounds very similar to DJ Octavio's voice sounds in the Octoling's communicator, Pearl points out that this voice "seems familiar to you". The character behind the mysterious voice does some kind of technological tweaking to make one of the building's floors available, which upon clearing it, will make more floors available. As the Octoling clears more floors, more will be unlocked as a result. However, this journey is not without its own set of dangers, there will be zombie-like Salmonids infesting the building, but you're not alone, as the mysterious voice will tell you about Octarians who are trying to survive, these Octarians are crucial to your journey to find the source of all this problem, as they have the technical knowledge to unlock the floors accessible via the elevator, so you are instructed to rescue them. Upon rescue, they will populate Inkopolis Square, so you can talk to them there, they will talk about how a song made them want to have a life on the surface, a story that resonates really well with the Inkadian Octolings who heard Calamari Inkantation.
After rescuing every Octarian survivor from the depths, there's one more floor to go, but unfortunately, it's inaccessible. However, the mysterious voice finally makes itself known to be none other than DJ Octavio himself. DJ Octavio knows the whereabouts of Marina, as he plans to help Pearl and the Octoling rescue her, he reveals that the Octarians have faced extremely hard times ever since power ran out of the domes, so he decides that the only place the Octarians can survive is on the surface. He will then unlock the final floor for the Octoling character to go to. A shock of realization comes upon the group when they find out that this whole plan was enacted by Marina, but something is odd about her, it seems she's not herself, regardless, she needs to be fought, there will be tons of machinery used against Agent 8 (or not-Agent 8) that Marina will activate, each one will be destroyed as Agent 8 (or not-Agent 8) splats each machine's core.
Upon a last desperate attempt to win this battle, Marina will release thousands upon thousands of the zombie Salmonids, but the Octarians rescued beforehand come in with their own machines and wipes out the zombie Salmonids. After Marina's defeat, she's finally herself again, after she and Pearl start to resume their friendship, something giant awakens, a giant mind-controlling skeletal fish that was revealed to be the true source of this problem. This fish starts mind-controlling the rescued Octarians, but DJ Octavio just busts in with "I Am Octavio" blaring loudly to counter the fish's mind-controlling effect, this is the perfect opportunity for the rest of the group to defeat the skeletal fish once and for all. Once the skeletal fish is defeated, the rescued Octarians have reclaimed their free will, Pearl and Marina are best friends again, and DJ Octavio says he needs to deal with a problem out there in space, but says he will return to help the rescued Octarians get accustomed to the life on the surface.
The Octoling we will be playing is definitely not the same as the playable Octoling we already have available for Splatoon 3, as the one seen in Side Order looks noticeably older. Pearl and Marina will be involved in the story in some way, their roles however, are unknown at the moment, but they will play some sort of important role in the story. Whether this is just in that Octoling's mind (I doubt it due to the presence of other characters' role in the story) or Inkopolis Square really did lose all color remains to be seen.
Now for the not-so-obvious details (with some speculations thrown in to make sense):
The official Splatoon Twitter said something like "getting to the bottom... or top of this". This is a possible hint as to what will be involved in the story, this brings me back to the idea that Agent 8 (or not-Agent 8) will be descending that elevator in the images to bring the Octarians to the surface, thus calling back to the tweet adding "or top of this."
Another detail that's not-so-obvious is the noticeable absence of DJ Octavio in the Return of the Mammalians story, where he only appeared at the beginning (and optionally through a memory) and at the climax of the story to help Neo Agent 3 defeat Mr. Grizz.
This makes me think that he will have a very important role in the story of side order, perhaps the same role as Craig Cuttlefish once had in Octo Valley and Octo Expansion. So this could mean that Side Order could take place at the same time as Return of the Mammalians, simply because by extension, Pearl and Marina would also have a good reason why they weren't involved either.
So with all of the details (and some speculation) out of the way, here's what I think the story of Side Order will be:
Agent 8 (or not-Agent 8) will first appear in a customization screen, with the text reading something along the lines of "Who am I?", the text itself will not change after you picked the character's gender, it will stay there as you choose further customization options, such as skin tone and eye color. After the customization has been decided, Agent 8 (or not-Agent 8) will wake up in Inkopolis Square, unsure of what happened that caused the place to be drained of color. He/she will then get a call from Pearl, which will be static-like at first, but will clear up as Pearl talks further about Marina's disappearance. After Pearl suggests that Agent 8 (or not-Agent 8) to first investigate the mysterious building that seems to have replaced the Deca Tower, the Octoling goes inside, which is revealed to be an elevator.
For now, none of the floors are accessible, with the investigation at seemingly at a dead end, Pearl doesn't know what to do, until a voice that sounds very similar to DJ Octavio's voice sounds in the Octoling's communicator, Pearl points out that this voice "seems familiar to you". The character behind the mysterious voice does some kind of technological tweaking to make one of the building's floors available, which upon clearing it, will make more floors available. As the Octoling clears more floors, more will be unlocked as a result. However, this journey is not without its own set of dangers, there will be zombie-like Salmonids infesting the building, but you're not alone, as the mysterious voice will tell you about Octarians who are trying to survive, these Octarians are crucial to your journey to find the source of all this problem, as they have the technical knowledge to unlock the floors accessible via the elevator, so you are instructed to rescue them. Upon rescue, they will populate Inkopolis Square, so you can talk to them there, they will talk about how a song made them want to have a life on the surface, a story that resonates really well with the Inkadian Octolings who heard Calamari Inkantation.
After rescuing every Octarian survivor from the depths, there's one more floor to go, but unfortunately, it's inaccessible. However, the mysterious voice finally makes itself known to be none other than DJ Octavio himself. DJ Octavio knows the whereabouts of Marina, as he plans to help Pearl and the Octoling rescue her, he reveals that the Octarians have faced extremely hard times ever since power ran out of the domes, so he decides that the only place the Octarians can survive is on the surface. He will then unlock the final floor for the Octoling character to go to. A shock of realization comes upon the group when they find out that this whole plan was enacted by Marina, but something is odd about her, it seems she's not herself, regardless, she needs to be fought, there will be tons of machinery used against Agent 8 (or not-Agent 8) that Marina will activate, each one will be destroyed as Agent 8 (or not-Agent 8) splats each machine's core.
Upon a last desperate attempt to win this battle, Marina will release thousands upon thousands of the zombie Salmonids, but the Octarians rescued beforehand come in with their own machines and wipes out the zombie Salmonids. After Marina's defeat, she's finally herself again, after she and Pearl start to resume their friendship, something giant awakens, a giant mind-controlling skeletal fish that was revealed to be the true source of this problem. This fish starts mind-controlling the rescued Octarians, but DJ Octavio just busts in with "I Am Octavio" blaring loudly to counter the fish's mind-controlling effect, this is the perfect opportunity for the rest of the group to defeat the skeletal fish once and for all. Once the skeletal fish is defeated, the rescued Octarians have reclaimed their free will, Pearl and Marina are best friends again, and DJ Octavio says he needs to deal with a problem out there in space, but says he will return to help the rescued Octarians get accustomed to the life on the surface.