The plot is simple. 1 guy plans an assembly looking for 11 other participants to go to "eternal sleep" together.
The rules:
1. They need to vote unanimously to "pull the plug".
2. If even one person is against it then they talk about it until it's unanimous.
3. Anyone can leave anytime if they don't wish to participate in the assembly anymore.
The movie is not about the mystery or the suspense. They are a big part of the movie but they're definitely not the main focus. The main focus are the each of the 12 individuals and why they are choosing to "leave the mortal realm". As the movie progresses, the stories of each individuals starts to unravel while they try to solve the mystery of the 13th individual and each story makes it even harder for the 12 to come to a unanimous decision. Some stories are deeper than the other of course but it also what makes each of the characters unique in their own way.
It's not a horror, suspense or mystery movie. It's a story about 12 people trying their best.

Great movie. Watched it a few years back and re-watched it recently. I was shocked to see a low rating on such a gem.
Good cinematography, good cast/acting, good ost. Solid watch.