This Is Us Season 4 Episode 17 revolves around Randall's fantasies about Jack continuing to live.

While talking to his therapist, Randall fantasizes that Jack didn't die in the fire and immediately afterwards, Rebecca tells Randall about William. Randall and Jack go off to meet William, who is thrilled to meet him. Jack doesn't like the idea of Randall spending time with an addict, but Randall points out that Jack is an addict too.

The rest of Randall's life is equally perfect, from Jack being present at the first dinner with Beth to Jack making the toast at Randall's wedding. William's cancer is cured and Jack is there for Rebecca when her cognitive decline begins.

The therapist interrupts Randall and makes him look at what he fears might have happened had Jack lived. Randall then has another fantasy in which everything is wrong. Jack and Rebecca fight about William, William denies Randall is his son, and Randall goes away to school without a word.

In this fantasy, Kevin is a successful realtor who is working with Jack and Randall is trying to avoid his family, until finally Jack calls him to tell him about Rebecca's cognitive decline.

The therapist helps Randall get some insights from this exercise, though Randall is resistant.

To find out what happens, watch This Is Us Season 4 Episode 17 online.