Rebecca thinks Jack is still alive in This Is Us wedding episode clip

On Tuesday night, This Is Us enters familiar territory for Kate — Weddingville — but for the rest of the family, there are lots of surprises, complications, and emotions headed down the aisle.

While Kate (Chrissy Metz) prepares not to renew vows with Toby (Chris Sullivan) but exchange them with Phillip (Chris Geere), Rebecca (Mandy Moore) is looking forward to her piano performance at the ceremony. As her battle with Alzheimer's intensifies, though, she does not seem to be having a good day.

In this exclusive clip from "The Day of the Wedding," after Randall (Sterling K. Brown) reminds her of the run-of-show plan, Rebecca reassures him that she's "never been more ready," but then casually adds: "I just can't wait for your father to get here." While Randall processes how to react to the fact that she thinks she's about to see Jack (Milo Ventimiglia), who died decades ago, Rebecca's current husband, Miguel (Jon Huertas) tries to diffuse the situation. "It's okay, it's just been a bit of a slow morning," he deflects. When Randall notices some medication on the counter, he and Miguel have a deeper conversation — and one that will intensify later in the episode.

"Naturally, as we're starting to get towards the end of the series, there's a bittersweet kind of element," says This Is Us executive story editor Jon Dorsey, who wrote the episode. "There's a sadness to it coming to an end. Plus, there's a natural built-in arc that we're going on with Rebecca and her illness. And all that is sad, but it's organic to life and to the show."

That's not to say that viewers should brace for a morose hour; in fact, the episode teems with frivolity and mystery, too, as Beth (Susan Kelechi Watson) and Madison (Caitlin Thompson) try to sleuth out whom Kevin spent the previous night with. "Expect a fun wedding that looks at some of the more fun story elements that a wedding can of provide without rehashing some of the wedding stuff we've already seen before," says Dorsey. "Look forward to some funny romance detective work by Beth and Madison. Look forward to seeing what the Pearsons have been up to five years into the future for us. And laugh a little bit in the midst of some tears and some sadness as we head to the end of this thing."

Meawhile, brace for a hair-raising (or is that hair-razing?) '80s flashback story in the episode as well. "Jack and Rebecca helped to show us that it never hurts to mix up the routine, never hurts to switch up the way we look," he says. "But be careful what you wish for."

You'll probably wish you'd tuned into the sixth-to-last-episode of This Is Us, which airs Tuesday at 9 p.m. ET/PT on NBC.

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