Fan theories are such wonderful creatures aren't they? A symptom of the wonderful communities that exist online, growing and growing and creating spin offs, parody, art and literature from the media they love.
And no-one is more aware of the influence of fan communities than Dan Harmon, co-creator of Justin Roiland's smash hit Adult Swim animation Rick and Morty.
A while back Roiland and Harmon revealed that they had originally planned to put a big twist into the narrative framework of the show, but decided against building upon this "secret" for they knew that the online fan communities would inevitably work it out at some point, which he described as "a really remarkable thing".
"You cannot write payoff-based TV anymore because the audience is essentially a render farm. They have an unlimited calculation capacity. There’s no writers’ room that can think more than 20 million people who can think about it for an hour a day.... You can’t do it anymore. You can’t try to fool the audience."
So there's a million and one things that this secret twist could be, but according to Dan Harmon someone on Reddit has in fact already worked it out. This doesn't really level the playing field that much in terms of guessing as there's a lot of viable theories out there on the Reddit message boards, but there's one that really stands out above the rest in its detail.
Have you heard of the Rick and Morty Master Theory?
The Rick and Morty Master Theory
The Rick and Morty Master Theory was developed by Reddit user Rickest_Rick, and it's one hell of a read. To get the full theory I recommend you check it out the archived thread via this link, but i'll describe a condensed version here.
So this theory uses the show-established premise that there exists infinite multiple universes each with their own Rick and their own Morty. It also draws heavily from S01E10: Close Rick-Counters of the Rick Kind, which featured Evil Morty (also known as Eye-Patch Morty) and the (mind controlled) Evil Rick.
Basically the theory explains how the Main Rick of the show - aka Rick C-137, aka the Rickest Rick - relates to Eye-Patch Morty.
Through looking carefully at the backstory established for both Rick, Eye-Patch Morty and the Main Morty of the show (referred to as First Morty or Pilot Morty), the theory argues that First Morty might not actually be Morty C-137, the Morty from the same universe as Rick C-137.
It's important to note that the Pilot Universe in which Rick C-137 and First Morty live is never established to actually be Earth C-137, so never established that it's Rick C-137's original universe. We do however assume that First Morty does hail from the Pilot Universe.
So Eye-Patch / Evil Morty appeared during Season 1 and - though it's never been explained why - he harbours a deep hatred of all Ricks (or of all Rick-Kind, you might say). Prior to and during the events of Close Rick-Counters of the Rick Kind Eye-Patch Morty has killed off twenty-seven Ricks and kidnapped hundreds of Mortys by which to use as a means to hide himself from the council of Ricks.
After the remote controlled Evil Rick is killed and Eye-Patch Morty is rescued by the council he reveals to the audience that he's been the one controlling Evil Rick and vanishes into the crowd of identical Mortys.
First Morty Is Not Rick C-137's Morty
Whilst Evil Rick is browsing through the memories of Rick C-137 we see him interacting with an infant Morty, but it's stated in the show that Rick C-137 left the Smith family household in the Pilot Universe 20 years ago and hasn't been back since.
Given that First Morty is fourteen at the time the show begins it stands to reason that it can't have been him in that memory, it must've been a Morty from another dimension. We again see Rick C-137 with a baby Morty in S02E05: Get Schwifty, in a photo on the wall of Birdperson's house. Morty asks Birdperson who the baby is, knowing that it can't be him.
The Master Theory posits that Eye-Patch Morty is in fact Morty C-137 - Rick C-137's original Morty and the Morty we see Rick holding as a baby and experimenting upon as a child.
The premise is that something happened between them - likely Rick's poor treatment of Eye-Patch Morty during the experimentation - that turned Morty C-137 into Evil Morty. This explains why it's Rick C-137 that Eye-Patch Morty is attempting to frame for killing the other Ricks in the first place.
There's a number of pieces of evidence which support this theory as outlined in the Reddit post and from contributors in the comments below, so check that out for the full story.