On "Ben and Kate" (Season 1, Episode 4, "21st Birthday"), the siblings mark Kate's 26th birthday by coming to terms with issues in their recent past.
Kate's (Dakota Johnson) 26th birthday is nearing, but she doesn't want a party. She wants to just have a sundae at her favorite ice-cream parlor. Naturally, brother Ben (Nat Faxon), his best friend, Tommy (Echo Kellum), and Kate's co-worker and friend, BJ (Lucy Punch) conspire to put together an event to make up for the fact that Kate never had a 21st birthday bash. But Kate's little girl, Maddie (Maggie Elizabeth Jones), inadvertently reveals the secret, and Kate makes Ben promise not to throw a party.
In order to make the festivities special, Ben calls up Molly, a high-school friend of Kate's. This doesn't sit well with Tommy, who says that Molly never paid back $15 she'd borrowed.
As Ben and BJ are picking up party supplies, they drive by the childhood home of Ben's ex-girlfriend, Darcy, whose wedding Ben had crashed in Episode 1. He notices that Darcy's parents are moving, and he says that he and Darcy had planned to live in her parents' house. They'd even planted a tree in the backyard. In one of Ben's typical oddball dreams, he'd planned to carve the tree into a canoe after Darcy died and row her ashes into the middle of a lake.
Kate looks more distressed than happy about her surprise party, until Ben reveals his biggest present: Molly. Meanwhile, BJ jealously watches the friends reconnect.
Upon learning that it's not a stranger moving into Darcy's childhood home, but Darcy and her husband, Ben makes the quirky decision to dig up the tree. BJ tags along, because she's tired of watching Kate and Molly. At the same time, Molly drags Kate out of the house to find a "man-shaped birthday present."
After BJ throws a rock through Darcy's window, she graciously invites them in for coffee. Ben admits he's upset that Darcy and her new husband are "living our dream." Gently, Darcy reveals that "I never saw us spending our life together."
To save face, BJ makes an impassioned, rambling speech, in which she calls Ben "this sad, freaky clown," claims to be his girlfriend, and gives him a big, embarrassing kiss.
Tommy, who had volunteered to drive the drunk Kate and Molly, reveals why he was so upset about the $15: He'd planned to use it to take Kate out.
When Molly borrows $40 from Kate, not telling her it's for drugs (which Kate promptly throws back at the seller), Kate tells Molly she doesn't want to hang out anymore.
In the coda, Ben and the full crew take Kate to the ice-cream parlor for a sundae. When Tommy offers to pay, Kate says that he should ask somebody out who appreciates how wonderful he is, "someone like me" and kisses his neck. At first, he's happy but then second guesses the moment: "What does that even mean?"