In the first season finale of the Flash, how did they know their plan would work the way it did?
(Spoiler alert for anybody who's even more behind in the show than I am.)
So I recently watched the first season of The Flash, and during the season finale...
...the Reverse-Flash reveals that he has a way to send Barry back in time so that he can try to save his mother from being killed. Barry & his team decide to make a deal with the Reverse-Flash and take him up on his offer. But they already know that some future event in which Barry goes back in time - to just about the point in time that they're planning to send him to - will be the very event that contributes to the death of Barry's mother. Cisco even assumes that there will be "three Barrys" when Barry goes back in time - little kid Barry from the past, time-travelling Barry from the future, and "current" time-travelling Barry from the present. But, given that the time-travel incident that precipitated the murder of Barry's mum hadn't happened yet at the point when everybody was debating whether to take the Reverse-Flash up on his offer...
...how did everybody know that this wasn't the very event that would precipitate the murder? How did the possibility of this being "that event" not even come up when everybody was debating whether or not to do it?
Best Answer
First off, it's important to note that they really have no idea if their plan will work, but at this point, we're not supposed to assume Barry is thinking clearly. The one guy in the entire world that Barry should least trust to tell the truth, has just given him something that he wants on a very deep emotional level, and so Barry is being unreasonably trusting. The rest of the team knows how much Barry's mom's death impacted him, so they're willing to go along with this plan just on the chance it might help. This, despite knowing full well how dangerous changing the past is.
However, one thing we do know is that Cisco is right about their being three Barrys, because we know exactly where the "third" Barry came from. He's from a time 10 years in the future, where Flash, Arrow, and a few other have formed the Justice League. We know this because Thawne's newpaper article and flashback scenes, if we piece them all together, tell us the story.
At some point in the distant future, Thawne is going to figure out how to time travel, and is going to travel back into the past, to a point where the Justice League is active, to try and kill The Flash. This is not present day, but 2024 -- in the original timeline, Barry didn't become The Flash for another 10 years or so.
When Thawne gets to 2024, he is unable to defeat The Flash, but he does figure out Barry's secret identity. At that point, Thawne travels further back in time, to Barry's childhood, and near-future Barry follows him there. It's this 2024 version of Barry that shows up at his own childhood home and fights Thawne, and watches his mother die. That 2024 version of Barry then presumably returns back to his future, in 2024, but leaves Thawne stuck in the past.
Thawne is then forced to accelerate the original timeline, and turn Barry into The Flash 10 years early. It's the 2015 version of Barry that goes back to "save" his mom from Thawne, where he sees the 2024 version of Barry fighting Thawne.
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