Number Five's (Aidan Gallagher) ability to time travel is the catalyst for season two. Without him, the Hargreeves siblings would've been destroyed in the apocalypse. After ignoring his father's advice in season one and getting stuck in the future, Five is finally able to return to the past after years of experimenting. In season two, he realises that he's been trying to time travel on too grand of a scale and it's this realisation that serves as the tipping point for his family's survival in the finale.