Supernova explosion screws up faster than light drives.
Supernova suddenly chops off a part of the galaxy.
Explosion effects spread outwards at speed of light.
Therefore no way to know about it before the effect reaches you.
People trapped in the nova system just die.
People on nearby systems are suddenly isolated from the rest of the universe.
They can send messages, but only to those already caught.
To the outside universe it is simply an incomprehensible Bermuda Triangle.
What warning do you get of supernova? Would the star be obviously about to nova, or would it need to be worked out from astronomical records later, or is it a sudden, surprising thing? Actually just an ordinary nova or flare would do.
How long does the expanding Bermuda Triangle puzzle the outside for?
Do ships jump in to try to find out what's happening, only to realize that they can't go back?
Do people go in on humanitarian missions?
Three part novel?
Outside society trying to work out why ships have stopped coming from a star system. Ships that jump in are goners.
Then the effect claims another system, the nearest to the first at lightspeed.
Maybe someone gets out? How? By the time they realise that they're getting no response it's too late? Only if someone sends a hyperspeed probe and it doesn't return can they realise that there's something screwy going on. Then they need to send a second. Then on a freakish hunch they can jump in a straight line away from the first system. But they still haven't seen the supernova!
Panic and evacuation in the second nearest system either way. Terror everywhere else.
How far will the effect spread?
Once they've worked out what's happened:
What will have gone on inside the boundary?
Perhaps someone's isolated without supplies somewhere?
Discovery that one can jump in, but never leave.
Mission is sent.
Description of the end of the world on a planet in the nova system.
Or if it's a flare, redo Inconstant Moon.
Description of the arrival of the wave front in the second system.
Description from the point of view of the people stranded without supplies. Efforts to achieve self sufficiency, even knowing that they're doomed in the medium term.
Arrival of the rescue team from Part One.
How do they save the day? Bring enough supplies to endure long term?
Bring enough to get through the crisis? How long will the effect last?
Bring cryogenic coffins to wait it out in?
Bring an awesome ramjet drive to escape in? It can't outrun the nova, but the effect may cease eventually? Alternatively you could go on a long loop to make time pass faster.
Has this already been done? It's nothing that couldn't have been done in the sixties and there aren't that many ideas.
Maybe there's a part four where a planet is awaiting its doom, sees the supernova light, but its drives are only screwed up, some engineering work can make them usable. Just after this is discovered, the escaping ramjet streaks through the system. Now they have to rescue it somehow. It will take ages to slow down on its own.
How does the rescue happen? I think you have to accelerate to match, take off just the people, and then accelerate back down to normal speed.
You're still left with an enormous pocket of trapped people. Maybe the outside universe, and most of the planets in the pocket go all mote in god's eye, but on one planet they manage to get control of their population, and are the only remaining conscious beings?
What happens to them when the effect subsides, thousands of years later?