Of all the Philadelphia stories to imagine, this is one few would see coming: a balmy summer night atop a double-decker bus ambling down South Broad Street with a pair of hipsters leading a sing-a-long to "Tomorrow" from the Broadway show Annie, quietly sung with thought and soul.

Then the situation got serious on Sunday in the Double Decker Music Series tour, a two-hour excursion that features two music groups in each of its two hours playing atop a bus that snakes all over Center City, South Philadelphia, and Fairmount. The improvisational electronic drone band JNPR (that's "Juniper" without vowels) began making inexplicably ethereal sounds as the bus made its way down Pine Street toward Society Hill.

The music became increasingly modern as the buildings got older and the bus passed second-story windows, curtains open in the hot summer night, inside activities there for the peeping. One townhouse had a movie in progress on the roof deck, projecting images against a blank wall.

"I think it was Bill & Ted's Excellent Adventure," said Allison Heishman, one of 30 on the tour and associate artistic director at Azuka Theatre. "There's so much going on in the city at any given moment. Good things. Bad things. People making dinner, having their Sunday rituals. This is one of the most exquisitely Philadelphia things I've done in a long time."

Exquisite? Philadelphia? How those two words come together captures the incongruities of beautiful music and some of the harder urban landscapes of Philadelphia that are inherent in the Double Decker Music Series. It's becoming a summertime Philadelphia tradition, with two more tours to come Sept. 18 and 27 (the latter is sold out).

Started by tour guide Sebastian Petsu in 2013 but now operating in collaboration with experimental music presenter Bowerbird, the bus loads up passengers and departs at 8 p.m. sharp, initially with a tour that's much in the spirit of guide books such as Weird N.J. Petsu's immensely likable manner is all his own. He dresses in the obligatory white shirt and tie but looks like he has slept in it.

His vibe is subversive. Consider his talk about Betsy Ross, who died in 1836: "The first story we have about her having sewn the first American flag is around 1870 from her grandson. Since then, we haven't dug up much new information on Betsy. But we have dug up Betsy herself on a few occasions. Her third resting place is over there next to the hot dog tent."

Passing the U.S. Mint, Petsu points out something you could see perhaps only from the top of an elevated vehicle: In the open blinds of a window on the upper stories is a mannequin facing out onto the street. "OK . . . keep looking . . . wait for it." he said. Fleetingly, a ghostly, almost militaristic figure appeared in the window.

"I don't know if it's there for security purposes or is some kind of in joke or a representation of some guy who was influential in the mint," Petsu said.

The musical component, which began after Petsu's Center City once-over, created something of a movie sound track to the urban landscape, transforming sights you've seen many times, though nighttime adds a dimension of mysterious transformation. Each turn became something else, as bushy-haired singer Vessna Scheff accompanied herself on ukulele with intimate understatement, and Koofreh Umoren added muted trumpet commentary. Their poetic original songs sounded like watercolor paintings set to music. Then came the 1964 Dionne Warwick hit "Walk on By" boiled down to its essence - played while rounding the Philadelphia Museum of Art.

"Taking in music at the same time as taking in the city creates one big theater," said Petsu, who initially dismissed the idea of live music on a bus as ridiculous. "The small confines of the performance space and the small onboard sound system means that no group larger than a duo can perform. We also skew towards experimental soundscape-based acts in the selection process but more melodic song-structured performers have also fit in well."

Speakers are positioned under the seats. Passengers can hear the music well, but those outside the bus cannot, preserving the almost private, inside-y air of the tours, which consistently sell out. In close quarters, singer Scheff admitted, she couldn't help hitting the microphone with her face and the rear bus wall with the back of her head. Tall buses sway a bit. And because the musicians face the back, they don't see potholes coming. "And then we'd hit one, and I had to try not to mess up," said trumpeter Umoren.

Both groups loved the acoustics when the bus went under overpasses. The JNPR synthesizer sound, deployed with keyboards and knobs amid affectionate glances between newly married members Jeanne Lyons and Peter Christian, was transformed under the Vine Street Expressway. Christian employs a synthesizer that plays only one note at a time - like a singer - though extra acoustic resonance made those single notes meld. The music is entirely improvised, so the swaying and bumping caused certain things to happen musically. "The bus was playing the music with me, " said Lyons.

The pay isn't great - $50 a musician - though it's attractive enough that when Bowerbird's Dustin Hurt put out the word this year in the music community, he had more than 100 submission in two weeks. There are other benefits to performing: Scheff had a delightful CD of a recent live concert, sold in a plain cardboard envelope for a you-name-it donation. Though the audience included everyday people - say, young visitors from Glassboro - there were also artists always scouting for new forms of expression.

Exciting? Scintillating? Oddly, during JNPR's quietest moments, the passengers were particularly still. No, this Philadelphia story wasn't some party on wheels. "It was such a peaceful experience," said actress Liz Filios, a multiple Barrymore Award winner. "I felt close to the city in such a unique way."

Double Decker Music Series: Tours at 7:45 p.m. Sept. 18 and 27. Bus boards at Fifth and Market Streets. Tickets: $27 (sold in advance only). Information: www.bowerbird.org or sebastianpetsu.com/doubledecker.