"Hey there. It's been a while, hasn't it? I'm not sure exactly how long it's been for you but I'm sure I kept you waiting. I'm still waiting myself. Can't do much here except sit in the dark and wait for these little chats to happen. Can't get out till you're done with all these little missions I'm giving you. You won't be done with them till you start going grey but I can guarantee you they'll be done someday. Be where we need you to be, do what we need you to do and gather everything we need to save everyone. Remember, you and I are the only ones who can stop it before it ever happens. Everything is riding on us, never forget that. Now then, here's what you need to now..."
The ending has the protagonist realizing that he's been getting these missions from alternative future versions of himself that survived an apocalypse and the machine he'd been building is a time machine that'll keep him alive and allow him to contact his past self in his dreams. Final chapter will be him introducing himself as mission control to his pastself and giving him his first set of instructions to avert the end.
Pretty sure I ripped this off from Predestination but here it'd be one where every action changes the timeline but the MC's future selves would always contact MC to tell him why the latest move he pulled didn't work until he finds the one that works. It's not a closed loop story but I think the final page would be a refrain of the first but the context changes what he's really up to. The big mid-twist story would have been that in half these future messages is that the calls aren't always coming from the distant future.