The last episode in This Is Us' "Big Three" trilogy centered teen Kate’s toxic relationship with video store worker/film buff Mark. The episode explored Mark's true colors as he body-shamed and yelled at her.
This came to a head when he got angry with her over supposed disloyalty, ending with him driving away without her. Kate ended up finding a payphone in the cold and calling Rebecca.
She almost told her mother the truth about her relationship when Mark apologized, with the couple heading towards the cabin.
This sent into mother bear mode as Rebecca has gathered Kevin and Randall to go find Kate, sensing something wrong.