The following episode delved into the fraught relationship between a mother and daughter, in the form of Rebecca and Kate’s tricky bond. And you could argue that Tuesday’s episode of This Is Us will bring you both of those themes.

In “Still There,” the NBC family drama unspools a story in the late ’80s about a strain of chicken pox that is running through the Pearsons. Perhaps it won’t be a fire that claims Jack (Milo Ventimiglia) but an unusually severe case of this viral infection? “It was totally chicken pox that did him in,” quips Ventimiglia. “Jack just rolled over and died.” Deadpans Moore: “Yeah, I wasn’t expecting that.”

The two stars say that one difficulty in pulling off this arc was in trying to help the show’s younger Big Three actors — Mackenzie Hancsicsak, Parker Bates, and Lonnie Chavis — understand what it feels like to have the chicken pox. “I guess because they have that vaccine nowadays, kids just aren’t getting chicken pox [as much],” says Moore. “They’re like, ‘What does it feel like?’ We’re like, ‘It’s kind of like having the flu, but then having poison ivy.'” Adds Ventimiglia: “But also, they’ve never had the flu or poison ivy. We have the healthiest set kids.”

The Pearson household is further thrown into disarray with the return of Rebecca’s mother, Janet (Elizabeth Perkins), who is pecking away at Rebecca (Moore) and others in the house. (Remember how little Randall complained about being asked by her to step out of family photos? This stay kicks it up a notch.) “She decides to visit out of the blue, during the snowstorm as well, so there’s no exit,” says Ventimiglia. “God bless Rebecca for having to deal with the things that her mother puts through, especially one particular issue that goes back to making our kids feel equal and welcome.”

Which brings us to the mother-daughter friction. (In a promo for the episode, we saw Rebecca confront her mother over racism.) Janet and Rebecca find themselves at odds yet again, though in different ways than Kate and Rebecca clash. Janet is a “pain in the neck,” says Moore. “I’m not living up to my mother’s expectations, but I don’t feel burdened by it the same way that Kate feels burdened by not living up to her perception of what my expectations are.”

In fact, the whole Pearson crew becomes united in the idea that they are not enjoying grandma’s stay. “No one wants to talk to her, everyone’s avoiding her because we’re not that close,” says Moore. “It’s actually kind of odd that she decides to take it upon herself to come and lend a helping hand when that’s never really been our relationship before. We don’t go home for holidays anymore. We’re our own little unit, the five of us.”

For all the drama here, the story line for the five Pearsons in this episode feels a bit lighter than where we’ve been seeing this season in the ’90s, with Jack facing down problems with drinking and with his marriage — and, of course, there’s that ticking clock when it comes to his death. “We’re living in 1989, which is a happier time for the family and the marriage so that lends itself to bringing a little levity to the episode,” says Moore. “But I think there are other story lines with other characters that are much heavier and are carrying more of the emotional burden of the episode.”

Speaking of those stories, Randall (Sterling K. Brown) and Beth (Susan Kelechi Watson) have their hands full with foster teen Deja (Lyric Ross), Kate (Chrissy Metz) seems to be very concerned with a big gig at a big bar mitzvah, and an injured Kevin (Justin Hartley) struggles while trying to keep it together on Ron Howard’s war movie.

To read what the This Is Us producers had to say about last week’s episode — including Kevin’s pills — click here.

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