In-System is a near-future science-fiction roleplaying game set in the Solar System in 2158. Its hard-science-adjacent setting imagines established human communities on Luna, at the Earth–Luna Lagrange points, and on Mars, alongside colonies, stations, and outposts in the asteroid Belt and around distant moons. Players can explore a society shaped by space settlement, mining, religious communities, advanced robotics, and powerful artificial intelligence. The setting features four major AIs—HELIX, ATLAS, LUNE, and TERRA—which influence human affairs while depending on relationships with people. It also introduces the Laser Highway, a new deep-space infrastructure with far-reaching effects on civilization.

The game uses a familiar skill-based 2d6 system associated with Traveller and Cepheus-derived rules. Characters have characteristics, skills, equipment, and backgrounds, and resolve most tasks by rolling two six-sided dice with relevant modifiers against a target number. The In-System resource includes guidance on character creation, skills, genetics, artificial intelligence, spacecraft, travel times, fuels, spaceports, religions, and world-building.