It is the 2250's. The solar system has been colonized. Luna and Mars were some of the first places with permanent settlements, but humanity has spread to live from floating cities in Venus' atmosphere to towns that hug the hydrocarbon lakes of Titan. Small mining outfits dot the thin asteroid belt; research stations float in solar orbit, and exploration ships push the boundaries of the Oort cloud.
Despite centuries of half-joking effort, nobody has cracked the secret of faster-than-light travel. 100 years ago, the Tilson Expedition embarked on a massive generation ship sent toward a star that probably had an Earth-like world. Their penultimate transmission revealed that there was no habitable world orbiting their destination, and they have altered course toward the beta site.
Nations, both new and old, vie for space in the solar system. Earth - the only planet with air we can actually breathe; air that's sitting there, for free, was constantly mired with infighting right up to the day it disappeared.
Observation telescopes reveal the Earth was there one day, and gone the next. It simply vanished.
Luna, after some unusual wobbling, continued along in the Earth's former orbit.
Now the many nations of humanity sit with limited vital resources, with tensions mounting and a deepening economic crisis. Water is difficult to come by. Oxygen is difficult to come by. Food is difficult to come by.
Some megacorps are making a killing, entire moons have declared independence, and remnant Earth-bound nations are scrambling to find out what happened.
Social media has adopted the term "Earthgone" to describe the situation.
Then the ultimate transmission from the Tilson Expedition came:
Tilson Expedition to anyone who can hear this. Monsters are out here, and they're coming your way. They found us - tracked us down like a deer. They toyed with us, took their time, extracted memories and our [transmission interrupt - reacquiring] -rnings in fiction; stories like interminable grating noises that never stop, seeping behind your eyes, in your thoughts, intruding like gnats in your ears, never-ending [transmission interrupt - reacquiring] Listen. Whoever you are, you can't let them live. Beware the ones who bleed. It's the first sign of the second coming and judgment day, when God is triumphant hand-in-hand with the devil whose BANG!
The transmission has continued unabated - not static, but silence.
Earth is gone. Will humanity be next?
Let's see what happens when relatively hard sci-fi nations meet eldritch horrors from beyond the cosmos! Who's with me?