SIN is set in a post apocalyptic world, devastated from the war between the United Nations (U.N.) and the New Earth Alliance (N.E.A.) over the last resources of the planet. After years of battling and technical advance in the weaponry sector, both sides made peace to avoid the complete extinction of humanity. Now the United Earth's Federation (U.E.F.) is ruling over the planet, but resources are even more rare now and space might be humanity's last hope.Everyday life on Earth is now mostly run by gangs, terrorism and a strict class system.
The manga follows the plot lines of the "Skull" gang, a group of teenagers, trying to sustain their small renegade village and a group of assassins.
SIN provides examples of:
- Aerith and Bob: Cruella and Alena, though Cruella could be a pseudonym.
- Bio-Augmentation: Alena mentions this for the upper class in a throw-away line. It's supposed to make them better suited for their tasks.
- Deadpan Snarker: Zerus. "Your auditory skills have served you well."
- Leet Speak: 137, Enyo's robot does this, going into the chat speak direction.
- Parental Abandonment: The whole of the Skulls gang are orphans.