The In-Between

The In-Between is the unique passage between all universes. Outside time and space, the In-Between acts to separate universes, above all in terms of identity. As such, it functions as a brake on change and innovation flowing from one universe to another. When Ellie's grandfather tries to encourage her to go on a mission through the In-Between to find a remedy for his wife's dementia, he has no idea how this natural resistance to change will thwart his plans.
So it's no wonder that he can't know that her role will not be limited to saving a loved-one's life, but rather to coaxing whole civilisations to change.