I'm in love with Gretl. And Brigitta.

So was the rest of the audience Thursday night at the Merriam Theater during a delightful, kinetic Sound of Music, on through next Sunday. They were ready to fall in love all over again, and this national tour is one lovable production. Surprisingly tight, too: Where the film has dead, gassy stretches, this Sound of Music has momentum and clarity, especially in a packed, jamming Act 2.

Ah, but Gretl, littlest of the little von Trapps. My goodness, is Sophia Massa (in her national tour debut!) cute. And active: This tiny trooper jumps up and down on couch or bed, runs around during tunes. The young  girls in the audience, and there were a lot, loved her almost as much as the grown-ups did.

And thickly bespectacled Valerie Wick is hilarious as Brigitta, the daughter who can't help telling the truth. Again and again, she drives the plot by releasing cats from bags, as when she informs Maria that the Captain is in love with her.

The adults were good, too. Lauren Kidwell as the Mother Abbess closes Act 1 with a sternum-shattering high note in "Climb Ev'ry Mountain." Jill-Christine Wiley is a girlish Maria with a deep Broadway accent, a pretty and precise voice, and enough acting chops to persuade us of her growth into womanhood. Mike McLean as the Captain is really a softie. When he first hears his children sing, he melts like an Eismarillenknödel left out in the sun.

Director Matt Lenz refreshes the show by bringing it back to its roots, adding back two tunes cut for the film – "How Can Love Survive" and "No Way to Stop It" – and dropping "I Have Confidence" (added for the film). He has the excellent taste, though, to keep the exquisite "Something Good" (also added for the film).

These changes underscore the difference between warm Maria and ice queen Elsa. In the movie, Elsa loses out mainly on emotional grounds. That soft-soaps it. Maria stands for folk traditions, folk arts, the ways of her people. Elsa is a corporatist, a CEO, an enthusiastic collaborationist with the Nazis. She and Max (likably played by bustling Jake Mills) agree: It's all about self-preservation. ("That's I!/ And I!/ And me!") Elsa drops the Captain when he won't go along.

After Act 1 and its salvo of great tunes – "My Favorite Things"! "Do-Re-Mi"! "The Sound of Music"! – Act 2 is cunningly structured, mostly with reprises, then the show-stopping "Edelweiss." It hurtles pell-mell, wedding/ Kaltzberg Festival/ flight from the Nazis, to a smiling ending. Throughout, the stage settings by Douglas W. Schmidt, incorporating projection, flying walls, and a couple of monumental mise-en-scènes, drew gasps of pleasure.

Look, the show will make you happy, it steps lively, the kids are irresistible, and the songs are tremendous. Take your family. Take your friends. Take that guy standing over there. Two guys behind me were singing and crying and laughing. They, too, had fallen for Gretl and Brigitta.

Theater

The Sound of Music

    • Through April 29 at the Merriam Theater, 250 S. Broad Street. Tickets: $40-$120. Information: 215-893-1999, kimmelcenter.org.