SAN FRANCISCO — An unexpected appearance by Joan Baez, impressive blues chops of Taj Mahal and punchy one-liners by Chris Isaak highlighted the Acoustic-4-a-Cure concert at the Fillmore benefitting the pediatric cancer program at UCSF Benioff Children’s Hospital—the first since before the pandemic.

The concert—hosted by Sammy Hagar with Bob Weir and including a rotating cast of contributors like Van Halen’s Michael Anthony, Chris Isaak, guitarist Vic Johnson, bassist and producer Don Was and Heart’s Nancy Wilson—felt like an communal jam session or an open mic night where no one was exactly sure what was in store next.

But it was Baez, who’d retired from performing live about half a decade ago, that came as the sweetest prize, about halfway through the show, which ran more than two and a half hours.

“I’d retired from music; hung my guitar on the wall,” Baez explained after being called up by Chris Isaak and duetting with him on a twangy cover of Delaney Bramlett’s “Never Ending Song of Love.” “This is my first public performance in four years. I wanted to see Taj Mahal.”

Baez then tore through the delicate “It Ain’t Me” by herself before stopping to collect herself.

“Ahh, the Fillmore. I used to not be able to see across the room! How times have changed,” she said. “Since it stopped being illegal, it stopped being fun.”

After getting situated on a stool, Baez worked through an intricate rendition of Bob Dylan’s “Seven Curses,” with Bob Weir joining in, followed by 2018 song “Another World.” Backed by her son, Gabriel Harris (who sat in on hand percussion for the first half of the show), she used her guitar as a percussion instrument on the latter song.

Baez closed out her mini-set with spiritual standard “Ain’t Gonna Let Nobody Turn Me ‘Round,” which was a cappella other than a drum strike here and there, joined by Nancy Wilson and, again, Isaak. But she returned to the stage several times afterward, dancing to the music or singing backup.

Taj Mahal came immediately after Baez and kept building on her momentum with a set of folky, melodic songs and his signature gravelly voice that brought on cheers every time he stretched it. The Bay Area bluesman didn’t diddle-daddle, keeping stage banter to a minimum and playing most of his set solo. The performance included “Dust My Broom,” “Corrina” (with Baez dancing next to him), “Stagger Lee” and “The New Hula Blues.” The crowd cheered for an encore, and Hagar came out to address the demands.

“Taj Mahal you can do whatever you want,” Hagar said, to which the bluesman obliged with “Cakewalk Into Town.”

Isaak and Nancy Wilson didn’t have as much stage time. Wilson was the first of the guests to perform, opening with Procol Harum’s “A Whiter Shade of Pale” with Vic Johnson playing lead on an electric guitar and backed by Hagar, who pointed at his arms as if his hair was standing on end at the iconic riff. After the two other artists walked off stage, she played Heart’s “Even It Up” solo, the guitar strings rattling on her acoustic guitar. Halfway through, Hagar returned, this time with Bob Weir, and the two added a bluesier element to the song.

After a cover of Peter Gabriel’s “In Your Eyes,” backed by Michael Anthony and Gabriel Harris on the congas, Wilson told a story of gifting Eddie Van Halen an acoustic guitar and him calling her the next morning to share what he’d come up with on it. She then played her 2021 solo song, “4 Edward With Love,” an instrumental, classical guitar tune.

Chris Isaak followed Wilson, backed by his longtime percussionist, Kenney Dale Johnson, on a hand drum. Hagar joked that the two were late flying in from Nashville, and not in a fashionable manner.

Their set opened with a snippet, of, possibly, “Blue Christmas,” before Johnson stopped Isaak and demanded, “let’s do the good one.” He meant “Wicked Game,” which slayed.

“The world was on fire and no one could save me but Sammy,” Isaak ad-libbed the words. He followed that up with a cover of Roy Orbison’s “Only the Lonely” before calling Don Was and Weir to the stage for “Baby Did A Bad, Bad Thing,” during which he delivered his signature high falsetto alongside growls. Weir riffed along while Was provided the heaviness on a stand-up bass.

Isaak also showed off his stand-up routine, noting how some in the audience were wearing face coverings.

“We both tested. We don’t have Covid,” he said. “Gonorrhea, yeah. But I don’t think you can get it at this distance.”

As the hosts, Sammy Hagar and Bobby Weir opened the show, provided backup to the other guests and then concluded it with a handful of songs and jam sessions. Flavortown mayor Guy Fieri also made an appearance, supporting his friends. Hagar opened the show by declaring that despite the show’s name, “I can’t play acoustic guitar for shit,” instead grabbing an electric for jam “Affirmation” and the slower, meditative “Father Time.” “Your Love is Driving Me Crazy” was the first rocker of the night, with Hagar declaring that he had to play a hit.

The end of the show, with Weir and Hagar leading the way, consisted mostly of Weir and Grateful Dead songs. Weir opened with the bluesy “Josephine.” About six minutes into the songs, one of the strings on his acoustic guitar busted, so he switched to an electric. Weir and a couple of other musicians jammed right along into the Dead’s “The Other One” and “Dark Star,” then “One More Saturday Night” (at which point Hagar joined him), a cover of Wilson Pickett’s “In the Midnight Hour” and back to the Dead for “Loose Lucy.”

To round things out, Hagar called everyone back to the stage for an a cappella rendition of his tune “Cabo Wabo,” with the audience joining in. Isaak handed Baez his sparkly suit jacket, and she enjoyed dancing along while wearing it.

Follow editor Roman Gokhman at Twitter.com/RomiTheWriter. Follow photographer Steve Carlson at Instagram.com/SteveCarlsonSF and Twitter.com/SteveCarlsonSF.