INTRODUCTION:
To put it bluntly, we don’t have much Story info to go on right now just breadcrumbs, we don’t have the whole loaf, not yet. That my fellow Halo fans will come sometime later this year, out of the oven that is 343 Industries. That said, ever since “halo For Xbox One” was announced, and later re-branded “halo 5 Guardians” 343 has let some crumbs fall off the table and into the hands of hungry fans awaiting below. So here they are, ready to be feasted upon. I don’t know about everyone else, but I’m getting hungry, let’s dig in.
Halo 5 Guardians Official plot synopsis:
“Peace is shattered when colony worlds are unexpectedly attacked. But when humanity’s greatest hero goes missing, a new Spartan is tasked with hunting the Master Chief and solving a mystery that threatens the entire galaxy.”
Quotes from 343 Industries and Halo voice actors:
“He is human, he’s always been human, but at the end of Halo 4 he really is reflecting on who he is and why he exists,” -Bonnie Ross
"It’s more about what he’s searching for versus what the UNSC is asking him to do."
-Bonnie Ross
“You won’t know what the purpose of finding Master Chief is [right away]. Or what will happen if we do find him, what condition he will be in, and where he’s at mentally. But I’m sure that’ll be a plot twist in Halo 5.”
-Mike Colter (Agent Locke)
“He is deliberately masking his appearance. And that is all you’re getting.” -Frank O’Connor
Visual Evidence:
A Covenant World, Sangilious?
Infinity above a Human colony World? This location is featured prominently in Halo 5. Remember, “Colony Worlds are unexpectedly attacked.”
Blue Team In Halo 5: This image is from “Awakening: The Art of Halo 4” and can be found on the last page. It clearly shows Fred, Kelly, and Linda. A hint of things to come? I think so.
Fred In Halo 5 Guardians. Note in this picture from Halo Catalog, the braille on the chest plate reads “104” Fred’s number. It is a comparison between Fred’s looking in Halo: Escalation and the Halo 5 render.
Misc. potentially plot-related quotes from across Halo media:
TN72H: John disobeys an order from Lord Hood to get R&R. John leaves with Blue Team on a mission. “Chief…refusing to stop. Taking one mission after another…something’s got to give right? He can’t keep doing this to himself for ever…yeah, but if he wants to destroy himself…hell, who’s going to stop him.”
H2A Prologue/epilogue: “He’s gone awol and the UNSC want him back…I’m going to bring him home.” -Locke
H2A Trailer: “Because all are lives are at stake.” -Thel
Conjecture/rumors/Theory**:**
I wanted to start the conjecture section by clearing up the whole “Halo 5 is going to be the darkest Halo yet.” Misconception. No one from 343 or Microsoft has ever said this about Halo 5. It was however, said about Halo 4. Here’s the source.
So there you have it, the origin of this statement. Halo 5 very well may be a dark Halo game, but there is no direct evidence supporting this.
Conclusion: I will add more story/plot related info as it becomes available. If you have any other sources feel free to link them and I’ll add them to the OP. Also, feel free to discuss your H5G story/plot theories.