Warning: SPOILERS ahead for Rick and Morty Season 3, Episode 9
Rick and Morty doesn't take itself very seriously, and since it's a science-fiction series (albeit with an anarchic comedy bent) Dan Harmon and the writers have always taken a particular joy in needling fans who try to take it seriously anyway. The show mocks its own sense of continuity, blows off seemingly "important" plot threads with a deliberate shrug, and uses its central sci-fi conceit (that the titular "Rick" is able to travel interdimensionally between alternate realities comprising infinite incarnations of every character - himself included) as both a fix-all for almost any extreme plot turn and a way of reinforcing Rick's omniscient-nihilist view of the universe as something like a series motto: "Nothing matters."
Except that it does, and that's the drama that's underpinned all the comedy ever since the fullest extent of what Rick's abilities (re: being the smartest man in the universe and also the only one who gets to break the "you only get one reality" rule) were revealed in season 1's "Rick Potion No. 9;" where a rare attempt by Morty to exploit his grandfather (Rick's) inventions (and lack of ethical restraint) to his own ends effectively destroyed the world. Rick "solved" this problem by casually relocating both of them to another reality - one identical to their own, save that the apocalypse had been averted and the other Rick and Morty had randomly died shortly thereafter. The concluding moments left Morty (and the audience) dumbstruck at how impossibly easy it was for them to "get away" with this and Rick's utter nonchalance; both numbed by the same chilling (to save nothing of mind-altering) question: "How many times has Rick done this?"
This, of course, is all part of the bigger (actual) mystery that sits at the foundation of the series' main plot: Rick Sanchez vanished without explanation from his daughter Beth's (Morty's mom) life when she was a young girl, only to re-appear decades later as a dimension-hopping alcoholic super-scientist who (for yet-unexplained reasons) "needs" to continue his various experiments - almost all of which involve sci-fi versions of small-time criminal activities. Why he left, what he was doing and why he's back now have all been questions whose answers are parsed out slowly, in pieces and often in contradiction over three seasons, but the overriding implication has tended to be that it involves "something bad" that Rick is either working to prevent or to bring about (season 3 has largely been all about reminding us that it's perhaps more likely that Rick is the villain of his own story), and that his coming and going are directly tied to this.
"THAT ACORN PLOPPED STRAIGHT DOWN"
But if Rick is not the most dangerous thing in the Universe, Rick and Morty has no shortage of possible other characters seemingly prepared to step into the role - if they aren't already occupying it. The Galactic Federation don't seem to be necessarily be good guys, and neither do The Council of Ricks (a collective of Rick's alternate selves from across the multiverse, who regard "our" Rick as the worst of the bunch) and multiple other occasional nemeses. There's also "Evil Morty," a memorable season 1 villain who made a shocking return two episodes back that seemed to reposition him as the new looming Big Bad. Indeed, the idea that "the enemy" (if there is one) could be an alternate version of a member of the Sanchez family (Rick, Morty, Morty's sister Summer, Beth and her now-ex-husband Jerry Smith) is a popular fan theory; and last Sunday's episode featured a shocking mid-story swerve that found one of them finally realizing - and excepting - that they were in fact "just like Rick." But it wasn't Morty (or even "a Morty")... it was Beth!
Titled "The ABC's of Beth," the episode sent Morty and Summer off to deal with Jerry in a comedy B-story while Rick (reluctantly) came clean to Beth about a few things. Specifically, after learning that a locally-infamous death row inmate convicted for murdering his son Tommy years ago was finally set to be executed, Beth recalled that Tommy had actually been a childhood friend of hers and that she had "convinced herself" at the time that he'd not been killed but rather lost in "FroopyLand," her imaginary fantasy world. Problem? As it turns out, FroopyLand wasn't imaginary: It was an actual place, a pocket-universe (designed in the manner of a My Little Pony/Lisa Frank rainbow-colored wonderland) that Rick had created for her as a girl - meaning that Tommy might still be alive and trapped there.
A rescue attempt led to some fairly disgusting (even for Rick and Morty) reveals about how Tommy had managed to survive and what had become of FroopyLand with Tommy as it's "king;" but that turned out to be misdirection for the big reveal: Every Rick, in every reality, also made a FroopyLand for "their" Beths; but not as she'd assumed to get rid of her - they did it for the protection of the neighborhood. As Rick puts it: "You [Beth] were a scary f***ing kid, man!" and "It was just more practical to sequester you before I had to start, y'know, cloning a replacement for every less-than-polite little boy or gullible animal that crossed your socio-path."
To prove his point, Rick revealed a both of toys that young Beth had asked him to invent for her - all of them either horrific variations on stereotypical "little girl" toys (a stuffed animal with biologically-correct organs for dissecting, a pink talking switchblade, a cat-o-nine tails "that makes people like you") or just practical tools that a serial killer might need - fake fingerprints, "rainbow" duct tape, a replica police badge.