The C17 Universe is the primary universe setting for City 17 Street. Although very similar to the main "Half-Life Universe", it does have major differences and takes elements from the original Half-Life 2 Beta.
Following Gordon Freeman destroying the Dark Fusion reactor in order to stop Wallace Breen, Nathan Johnson got caught up in a malfunctioning portal which sent him back in time. In order to prevent the incident from happening, he delayed Gordon Freeman's train which caused major changes. In an attempt to keep things stable, the mysterious Elders created a new parallel universe, which despite being very similar, does have significant changes. This includes:
- The span of the Combine's occupation of earth only being at least several years, rather than twenty-years like in the original timeline.
- Whereas the events of Half-Life 2 took place in the 2020s, Gordon Freeman's arrival in this new timeline began in "present" moment in the mid 2000s.
- Children existed during the Combine occupation, whereas they were eradicated during the original timeline.
- Wallace Breen didn't die at the top of the citadel. The citadel was also spared, meaning City 17 wasn't destroyed like in the original timeline.
This new timeline lasted for many years through Nathan's perspective.
The mysterious Clandestine Company plotted to takeover this universe and many others, and built a deadly teleporter known as the "Dark Portal". Nathan destroyed the portal, but caused this timeline to become unstable and crossing over with different universes, including one known as the "Real World Universe" (in which inhabitants from the C17 Universe and HL Universe are fiction). A mysterious force ended up merging the two universes, shifted things around, which gave birth to the "New Universe".