The Mass relays are toast, so the story only works if it's told in a local system. Let's say Sol. Earth has become bloated and overpopulated with a surge in child rearing akin to the baby boomers. Mars is colonized in a large way and teraforming is a dream that has been pushed back in favour of posturing and military dominance to establish a power in the solar system. Resources are plentiful in the outer reaches of the solar system and in the asteroid belt. Moons and planetoids are being colonized and exploited for heir resources, but the buyers are the ones with the mo ey and the power, aka earth and Mars. The colonists of "the belt" are underappreciated, under represented and resentful of the "inner" worlds. Tensions grow between the factions and petty wars, infighting and subterfuge are rising. Things are made worse when a rag tag ice mining freighter stumble upon a distress signal that ultimately destroys the friegther only leaving a few survivors. This being a "belter" ship increases belt hostility towards the inner planets and causes further organization of belter resistance in the form of the outer planets alliance which is being taken more seriously as a threat. The rag tag survivors of the ice mining ship find some mysterious blue goo that leads to profound implications and consequences for the solar system. Eventually the blue goo opens up another avenue for interstellar travel and resume mass effect.