A few episodes ago, Kwan's aunt told her about her father's pilgrimage to the Madrigal desert, where he encountered and joined a group of mysterious people. Pretty vague, right? Well, upon his arrival back home following this, he was a completely changed man, though for the worse. After hearing this, Kwan sought to find the possibly-fictitious people in the desert who caused her father to change. Given the lack of details about the group's whereabouts, it seems like it would be pretty tough to locate them. Wrong.
After driving into what looked like the Dust Bowl circa the 1930s, she happened upon a group of people in silhouette and decided to get out of her car to check out the scene (always a good idea). A few seconds later, she's kidnapped and brought back to the group's campsite.
She quickly realizes that these people knew her father and actually weren't awful people (you know, the ones who usually carry out kidnappings). They're a bit sketchy, though, and the main woman of the group, Desiderata (Josette Simon), pulls an Oprah-walking-on-hot-coals moment and lights a flame in her hand which she turns into water, which Kwan is told to drink.
She's immediately brought into an altered state of mind, where she encounters her deceased relatives as well as Master Chief, whom she ends up fighting as part of a weird test. Try as she might, she's basically attacking a brick wall when it comes to approaching him. She uses her fists, a knife, a gun, and even a large rock to defeat him, but each time, he kills her, bringing her back to the start and repeating the process once again.
Though she wasn't ever able to defeat Chief, she was able to face her father, as Chief gave her a hand and led her to him. There, she was able to ask him what she should do with her life as Vinsher is now in control of the planet. To that end, he tells her that she is ready to protect Madrigal and clues her in on a secret portal in the desert (though she's not quite ready to know the full details of it just yet).
Upon being brought back to the present day, Kwan realizes that her father actually wasn't the terrible person that her aunt was making him out to be. What he actually encountered in the desert was a profoundly introspective experience, as she has now gone through the same thing herself.