The OS the people who created Unix did after they'd decided that networked workstations with high-resolution graphics displays and mice were the way the future would look. In some ways, it's a cleaned-up Unix, and in others, it's a different design, based more around computers on a LAN sharing resources than than terminals connecting to a central system.
In a nutshell: an iterative improvement on the UNIX concept of everything is a file. In UNIX everything is a file unless it isn't (a device, for instance, which just looks like a file but does not behave as such), in plan 9 that is so cleanly implemented that you can run one window manager inside another (and if you want while using the CPU resources of yet another machine) without losing stride.
You can also watch an excellent series of videos[1] from the adventuresin9[2] YouTube channel, showcasing the Plan 9 system itself (based on 9front fork) and its capabilities.
https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs