Jon Negroni thinks that every Pixar movie is connected. I’m unconvinced, but it makes for a fun theory!
In Brave, Merida discovers that there is “magic” that can solve her problems but inadvertently turns her mother into a bear. We find out that this magic comes from an odd witch seemingly connected to the mysterious will-of-the-wisps. Not only do we see animals behaving like humans, but we also see brooms (inanimate objects) behaving like people in the witch’s shop.
The Pixar Theory
And so begins a timeline that sees animals (Finding Nemo, Up, Ratatouille) and artificial beings (Toy Story, The Incredibles, Up) become more and more sentient and more in conflict with the humans, who are later forced to leave Earth on the Axiom (ref. Wall-E).
In their absence the era of the Cars begins. But they throw the balance off even more, wiping out animal life on Earth.
Then comes the events of Wall-E, then A Bug’s Life (which is placed here in the timeline for interesting reasons), and then – even further in the future – Monsters Inc.
In a final twist Negroni connects the Monsters Inc. time to the distant past of Brave in a very interesting way that I won’t spoil here.
Like I say, I’m far from convinced. I just don’t think movies are written this way. For example, I know that the plot of Ratatouille was rewritten very late into production. These kinds of story considerations come before grand ideas that most people will never notice. Most of the other references (BnL batteries in Toy Story 3 for example) are nothing more than in-jokes for the fans to spot.