This Is Us: Mandy Moore's Rebecca Learns Her Diagnosis as Sophia Bush Makes Her Debut

This Is Us is picking up right where it left off after the season 4 fall finale — with two new faces.

For a brief period of time, in present-day, all the Pearson family members are in Los Angeles as Randall (Sterling K. Brown) makes a secret trip to the West Coast on a mission to find his mother Rebecca (Mandy Moore) the best neurologist for her ongoing memory loss, which has been preliminarily diagnosed as mild cognitive impairment.

Meanwhile, Kate (Chrissy Metz) confronts husband Toby (Chris Sullivan) about his CrossFit ally LadyKryptonite5, whom he has been confiding in behind his wife’s back. But Kate attempts to turn over a new leaf by throwing him a Jimmy Buffett-themed surprise birthday party in hopes of getting to know his workout buddies and meet the mysterious female friend.

Then, there’s Kevin (Justin Hartley), who is determined to find a partner, even shamelessly hiring a matchmaker. But a spontaneous trip to a local coffee shop finds him locking eyes with Lizzy (Sophia Bush) for what appeared to be a perfect meet-cute.

RELATED: This Is Us Creator Dan Fogelman Teases What’s to Come for Each Character in the Rest of Season 4

However, This Is Us fans are already very familiar with how big twists in storylines can make things complicated, both in past and present. Such is the case for the midseason premiere, “Lights and Shadows.”

The episode explores Rebecca and Jack’s early romance as boyfriend and girlfriend. After Jack’s disastrous (and boozy) golf endeavor with her dad, he takes her father’s discouraging words to heart when he breaks up with Rebecca for “being too different,” especially their contrasting financial backgrounds. And in what appears to be Rebecca’s first step of defying her father, she rushes to Jack’s job site to tell him, “I love you” first, and to give their romance another try.

Back in the present day, Kevin pulls out all the stops, Bachelor-style, for an unforgettable date after learning that Lizzy was only visiting L.A. from Chicago. Using his Hollywood connections, Kevin organizes a private concert by hitting up her favorite singer, John Legend, to perform for them. The romantic gesture lands Kevin a first kiss with Lizzy, but she quickly confesses that she’s a married woman who indulged in fulfilling a fantasy celebrity hall pass. However, all hope may not be lost for Kevin in the romance department as his ex-wife Sophie gives him a call, seemingly out of the blue.

Another Pearson sibling whose love life is going downhill is Kate, who learns from Toby’s friend Kurt that her husband hasn’t been attending CrossFit classes for two weeks. If learning that detail from a stranger wasn’t hard enough, Toby confesses that LadyKryptonite5 came onto him with a kiss after he and Kate returned from their Thanksgiving holiday in Philadelphia. Trust was already broken between Kate and Toby, but it’s further crushed after he admits that their son Jack’s vision impairment has turned his mood at home into sadness.

RELATED: Sophia Bush and Pamela Adlon Join This Is Us Season 4 in Mystery Guest Roles

Though the truth comes out for his family members, Randall is still keeping things in the shadows. Along with keeping Rebecca’s diagnosis a secret from Kevin and Kate. The councilman is also having to deal with the stresses of his new job. Though his dedication to Rebecca is shown through his 47 emails to the L.A. neurologist, his need to prioritize his own mental health is foreshadowed for next week’s episode.

The burglary cliffhanger sets up the episode, titled “A Hell of a Week: Part One,” which is the first of another set of Big Three-centric episodes. The first of three episodes will focus on Randall and how he deals with his anxiety after coming home from L.A.

“Fogelman and our writers came up with a storyline that I think is quite compelling. It allows us to further delve into Randall’s mental health and how he takes care of himself — what’s right and what’s wrong about how he takes care of himself and how he could potentially do a better job of that,” Brown recently teased. “This moment happens where you’re like, ‘Oh. Oh s—, this is going to go sideways.”

This Is Us airs Tuesdays at 9 p.m. on NBC.