At first blush, the protagonist in the new musical “Redwood,” Jesse, suggests one of those unbecomingly girlish middle-age women who might have been played on screen by Goldie Hawn years ago, or more recently by Sarah Jessica Parker. Here she’s portrayed by Idina Menzel, returning to Broadway for only the second time since winning a Tony Award more than 20 years ago, for her performance as Elphaba in the original cast of “Wicked.”

Like that green witch, Jesse longs to rise above the earth: not by flying in this case, but by climbing a tree. Her journey into the forest is preceded by a dizzying opening number — crafted by a young composer making her Broadway bow, Kate Diaz, and a musical theater veteran, Tina Landau, who conceived the show with Ms. Mendel, wrote its book, and co-wrote the lyrics with Ms. Diaz — that gives us background on Jesse.

We learn that our heroine is a lesbian with a longtime partner, Mel, and that the two are having some issues. It’s disclosed that Jesse works at an art gallery; she’ll eventually have one of her own, representing female artists and ensuring they receive “gender parity pay.”

Jesse is also a mother, and it’s this role that ultimately gives “Redwood,” despite its shaky start and enduring sentimentality, an abiding poignance. Upon arriving in the forest, Jesse meets a pair of botanists who are there to study carbon sequestration. Finn is the grizzled veteran running the operation; Becca is the feisty upstart, a Ph.D. graduate from Berkeley who laments the greed of “generations of white cis male corporations.” 

Finn proves more amenable when Jesse expresses her wish to tackle the mighty redwood tree. “I’m in great shape,” the inexperienced climber sings, adding, with pronounced perkiness, “I used to do ballet. I can hold a tree pose. I can do a downward dog, ’cause I did yoga once at my old synagogue.” 

The production, directed by Ms. Landau, makes it easy to see why Jesse would feel so drawn to nature. Jason Ardizzone-West’s lush set and Hana S. Kim’s swirling video design evoke a magical refuge from the stresses of Jesse’s everyday life, which are not all mundane: It turns out that a terrible tragedy has driven her away from home, and from Mel.

Both the musical and Ms. Mendel’s performance grow more compelling as that tragedy comes more sharply into focus. Ms. Diaz’s score, which seems influenced by various pop singer/songwriters who have espoused girl power in recent decades, gives the star a number of showcases for her trademark belt as she gradually reveals the depth of her character’s pain.

De’Adre Aziza’s spunky but grounded Mel proves a fine foil, as does Khaila Wilcoxon’s sharp-witted, mighty-voiced Becca. Michael Park brings a gentle, folksy grit to the part of Finn, whose relationship with Jesse might have become romantic in another musical but is propelled here more purely by empathy.

Zachary Noah Piser has a smaller role, as Jesse’s son, Spencer, but the rising star, whose credits include “Wicked” and the title role in “Dear Evan Hansen,” contributes a big voice and presence, handily carrying the show’s final and most affecting scene, right before its finale.

By then, whatever initial misgivings it may have produced, “Redwood” is likely to have wormed its way into your heart.