“Evil” seems like a redundant title for a Rick, but there are far worse ones out there than the Rick we've been following. If that isn't a scary thought, then I don't know what is.
The Council of Ricks arrest the Main Rick (of Dimension C-137) for supposedly murdering twenty-seven other Ricks throughout the known worlds. Rick suspects foul play, but the Council refuse to believe his innocence due to his rebellious nature. He escapes with his Morty, vowing to find the jerk who framed him.
Rick locates the perpetrator's base on a faroff planet. There lies a domed base covered entirely in screaming, eternally tormented Mortys. Because Ricks have a unique, traceable brainwave, they need a Morty to cancel them out. Using hundreds of them have kept the council off this planet. Inside the base lies the mastermind behind it all: Evil Rick.
Evil Rick wanted to lure Rick in to suck up the contents of his brain, leaving him as the twenty-eighth victim. Fortunately Morty saves the day, having rounded up hundreds of imprisoned Mortys to fight back. Believing him to be the "One True Morty", the gaggle of teens stampede the base and execute Evil Rick.
Rick summons the council afterward, pointing to the true killer. The council apologizes, sends the other Mortys home, and investigate the crime scene. To their surprise, Evil Rick had been remotely controlled the whole time, but they have no idea who did it.
Turns out the entire plan had been fabricated by Eyepatch Morty. He had been the puppeteer, using a remote lodged behind his eyepatch to control Rick. Why did he do it though? It's a mystery yet to be solved. The renegade teen is still at large and who knows when he'll strike. An Evil Rick is scary, but predictably so, but a diabolical Morty is something no one could have ever suspected and that may be a more frightening prospect.