WARNING: This article contains spoilers for the Season 4 finale of The Flash, "We Are the Flash."
Over the course of The Flash's fourth season, a mystery girl (played by Jessica Parker Kennedy) made a number of sporadic appearances where she would stumble upon and interact briefly with one member or other of the series' cast. Each of these appearances brought a bit of curious information, including the fact that she had super-speed powers highlighted by purple lightning, just like Iris West displayed during her short tenure as a metahuman hero. Finally, in the closing moments of the Season 4 finale, her identity was revealed, confirming a theory fan had from the first time we met her: Her name is Nora West-Allen, and she is the future daughter of Barry and Iris.
However, it doesn't seem like Nora reveals her identity by choice. Instead, after alarmingly introducing herself, she quickly adds, "I think I made a big, big mistake." This is the cliffhanger the series left us with, meaning that her "big mistake" will most likely be the driving force of the plot at the start of Season 5. So what is this big mistake, and what will it take to fix it?
During the season's climactic battle between Team Flash and The Thinker, the heroes managed to defeat the supervillain -- but not before his fail-safe to destroy Central City was activated. The S.T.A.R. Labs satellite came crashing down towards the city, a hail of flaming debris wreaking havoc. The main chunk of the satellite was large enough to destroy the entire city -- and more -- which means that the Flash had no choice but to run at super-sonic speed and deliver a massive punch to destroy it in the sky. However, when he tried to do just that, time froze to a standstill before the punch could be delivered, and wound back to when Flash started his fateful run towards the satellite.
When he began to run on this second go, the Flash was followed by a flash of purple lightning. Nora was now there, and she also delivered a punch to help destroy the satellite before it would crash into the city. Since the episode took the time to show us the bulk of The Flash's first attempt, followed by a trip back in time that culminated in a second, successful attempt with the help of Nora, it appears as if this is where the big mistake can be found.
This a moment that Nora wasn't supposed to interfere with, and yet she did it anyway. Why? Was it to save Central City, or simply to fight beside her father?