STRANGE CELEBRATION: A birthday party in mourning.

Complicating the grieving process for responsible, upper-middle class Becca is the news that bottle-blonde, leopard-print Izzy, notorious for scandal and excess, is now pregnant herself. The timing, as Izzy concedes, “sucks.” Plus, these two sisters are your classic sororal Odd Couple: They know and love each other intimately, but are about as opposite as dramatically possible. Izzy’s brassy Noo Yawk accent is glaring against her sister’s cultivated voice, and when they talk, their exchanges are chockablock with criticism — barely veiled (and sometimes stark naked) needling, judgements, and accusations about each other’s characters and behavior. Both the script and performances of Poirier and Kimball make the sisters’ tension acute and absolutely delectable. They are a total hoot. “I like how it oozes,” Izzy says of Becca’s perfect crème caramel. “Of course you do,” Becca replies, with just a dollop of subtext.

Becca and Howie, naturally, suffer from an even stronger tension, and are also far less available to each other as sources of comfort. They are completely out of synch in their respective grieving processes — Howie goes to group therapy, watches old home videos, and wants to keep Danny’s room intact; Becca quit therapy, keeps busy boxing things up, and wants to sell their now too-big house (Craig Robinson’s open set does a nicely understated job of suggesting their unflashy affluence and quiet good taste). Poirier and Rubin play off each other powerfully, and make their growing disconnection painfully tangible. Their characters’ individual arcs of grief, beautifully wrought by both actors, progress at different speeds and rates of change, never meeting in a common place, and their scenes together are hard on the heartstrings.

Luckily, all this misery is tempered by some exceptionally funny family comedy. Each woman of this family has a very sharp, and very different, comic sense: Becca is rather arch, Izzy brash and take-no-prisoners. And then there’s Nat. No-nonsense, simple, outrageous, and insufferable, especially once she’s had a few glasses, Mom rambles on in comic-relief tirades that any character actor would die for. What a pleasure for us that she’s in the hands of the wonderful Tootie Van Reenen, who’s heightened Nat’s outrageousness with a smart dead-pan and killer timing. Her extended treatise on the many Kennedys who have died in or escaped from airplanes (“The Kennedys are not cursed. They’re just really unlucky. And kind of stupid, a lot of them.”) is worth the price of admission. But she also does a marvelous, subtle job of balancing Nat’s intolerable qualities with ones that are matter-of-fact, wise, and empathetic.

The story also includes one character, Jason, who’s related to these folks by neither blood nor marriage: He (Jesse Leighton) was the young driver who hit Danny. He re-enters their life against their will, with a goodwill that’s rather blank and other-worldly. Indeed, the kid writes science fiction, and it’s from one of his short stories — a tale of searching through parallel universes down in tunnels — that this play takes its name. Jason’s presence helps highlight the ways in which Becca and Howie, themselves, seek and hide from each other. There is, finally, hope to be found in each other’s desperate outlets — even in chocolate tortes and lemon squares — once they know where to look.

Megan Grumbling can be reached at mgrumbling@hotmail.com .