Us: What Does The Ending Really Mean?

2. Jason Knows The Truth

Back to the narrative implications of the final twist, the question remains just what happens to the family after the Untethering? All the way through Us, Jason grows increasingly suspicious of his mother. Unlike the other characters, he catches glimpses of her echoing the behaviour of the the Tethered - extreme brutality, devolving into grunts and animalistic tendencies - growing to suspect that Adelaide might not actually be who he thinks she is.

This culminates in the final scene, where she remembers that she's actually from the underground and was the one who put Red there and stole her life. Jason doesn't know for certain, but his worried look as Adelaide lets out a sinister grin before the movie cuts to black indicates that he's put two and two together.

In fairness, it wasn't exactly a deep mystery to figure it out, and in hindsight the clues were always there. The fact that Red is the only Tethered who can actually speak (with the reason for her broken, coarse voice due to her being choked by Adelaide as a child and spending the next few decades not speaking at all), that Adelaide's parents noticed a change in her, that she confessed to having trouble speaking, it all paints a pretty obvious picture looking back.