Joss Whedon is a prolific creator whose writing credits stretch all the way back to 1990 when he wrote four episodes of Roseanne. Since then, he's penned scripts as diverse as Toy Story and Alien: Resurrection, had a successful run on Marvel comics' X-Men and The Runaways, as well as both the 1992 Buffy The Vampire Slayer movie and the beloved TV series of the same name. Oh, and he worked on a little side project known as The Avengers.
You'd think such a big body of work encompassing so many different worlds, characters, and mediums would be hard to connect - and you'd be right. But determined fans have found a way to do it anyway. This is largely thanks to the Easter eggs and repeated motifs in Whedon's writing - secret organizations manipulating things from the shadows, supernatural apocalypses, sci-fi experimentation gone wrong, and of course a strong female lead to save the day.
Pulling all this together, fans have come up with a proposed timeline that connects Buffy the Vampire Slayer, Angel, The Cabin in the Woods, Firefly, and even Agents of S.H.I.E.L.D. to create the Whedonverse theory.
In the 1990s, Buffy Summers becomes the latest in an ancient line of women chosen to guard the world from monsters. Those monsters usually takes the form of vampires, demons, and other things that go bump in the night. Buffy carries out her slaying duties well enough to avoid every arising apocalypse thrown at her by the Hellmouth that her town just happens to sit on.
Ultimately, the battle against the First leads to every potential Slayer being activated at once, breaking a chain that has lasted for thousands of years.
Buffy's longtime vampire beau, Angel, leaves her in search of a calmer life in Los Angeles. Instead, he finds himself swept up in more supernatural shenanigans with Wolfram & Hart, an interdimensional law firm. The firm is run by a group of powerful demons: Wolf, Ram, and Hart, collectively known as the Senior Partners.
The Senior Partners operate in secret through the Circle of the Black Thorn. When Angel and his crew destroy the Circle, the Senior Partners are finally lured out of the dimensional shadows.
Because the existence of other dimensions is firmly established in the Buffy and Angel TV series and accompanying comics, it's safe to theorize that Dollhouse takes place in a parallel dimension. On this alternate Earth, another shady organization called the Rossum Corporation runs "Dollhouses" that wipe people's minds before sending them off to rich clients to fulfill their fantasies.
One of these "Dolls" is Echo. She might not have a clue who she really is, but we do - she's this dimension's parallel Dark Slayer, Faith. Like Slayers, Dolls even have their own Watchers - or Handlers - but with their real identities taken from them, their purpose has been interrupted by Rossum.
Back over in the main universe, things become desperate now that the Senior Partners are alive and kicking. A new initiative run by a shadow government operating in various facilities around the world use a sinister system disguised as every horror movie cliche to appease them.
These facilities are likely run by the combined remnants of Buffy's Watchers Council and the Initiative. The American-based facility audiences follow in The Cabin In The Woods could even be what's left of Sunnydale's Hellmouth, with each similar facility positioned over other portals.