Like everything else in the world today, the event was captured on a cell phone camera and posted almost immediately on YouTube. Watching it, you can’t help but think of Jack Ruby as the man charges out of the crowd toward his victim, except Jack Ruby wore a fedora and a suit and this guy wore a white T-shirt and plaid shorts slung low.

He knows whom he’s after — Henry Kiely, who sings bass while standing on the harbor bulkhead, flanked by eight fellow singers, all juking to their signature song, Good Old A Cappella. The man drives Henry off the wharf with a two-handed push, the kind used by a second-grader on a playground. Henry flies back, his head snapping forward, just missing the bow of a Hatteras cabin cruiser called Tuff Ship, which is tied to the bulkhead, and he disappears with a splash into the Oak Bluffs harbor. The last thing the video records is a shocked “Oh!” from the bystanders and the attacker turning away from his deed.

Within moments, witnesses say, the assailant sensed that the crowd was not with him and decided to flee. As fellow members of the Vineyard Sound and one or two others pulled Henry from the harbor, the assailant ran up the walkway toward town, seven or eight angry a cappella afficionados in hot pursuit. After a chase through the Camp Ground, two young men corralled him on Circuit avenue. Blows were exchanged, a pair of eyeglasses on a pursuer broken. The man freed himself, continued up Circuit avenue, this time almost running into the arms of a police officer responding from the opposite way.

And with that, Casey Eason, 28, of Oak Bluffs, was arrested, charged with disorderly conduct and two counts of assault and battery, and taken to the Edgartown house of correction.

The Vineyard Sound, an all-male college a cappella group, got underway in the summer of 1992 and now performs more than 80 hourlong shows a season. This year the group drew its membership from five eastern colleges and sings its final concert tomorrow night at the Old Whaling Church in Edgartown.

From the e-mail messages that quickly filled the Vineyard Sound inbox from alumni around the country, some expressing relief that Henry escaped with minor injuries and others cracking jokes that cannot be repeated in a family newspaper, it’s clear that the shove of Monday night was unprecedented in group history.

The assault occurred during the last few minutes of a routine tour of street corners and waterfront restaurants, intended to tempt visitors to the regular Monday night show at Trinity Methodist Church on the Camp Ground.

At roughly 7:45 p.m., the singers were standing in a semi-circle, their backs to the harbor, between the Coop de Ville and Shipwreck restaurants. Eddie Silva — recently graduated from the University of Connecticut and serving this summer as the group’s business manager — exhorted the crowd of about 60 people to head up to the church for the 8 p.m. show.

Ship Wreck co-owner and hostess Leslie Graham was standing in front of the restaurant. She saw a man facing the singers, smoking a cigarette in an agitated manner. As Eddie touted the upcoming show, the man shot forward at Henry and pushed.

As Henry fell, he hooked a dock line under his arm, which kept his head from clipping the bow of the yacht.

As the group and a few bystanders hauled Henry from the harbor, Leslie Graham confronted the attacker, “shoving him in the chest with my hands, saying, ‘I can’t believe you did that! That was the stupidest thing I’ve ever seen anybody do! You know what I’m going to do? I’m gonna call the cops on you.’” The assailant decided to ditch his flip-flops and leg it for town. Leslie and a waitress called to the crowd ahead to stop the fugitive, who ran across busy Lake avenue and into the Camp Ground.

Back at the scene, Henry had strained his back and a rope burn ran so deeply under his armpit that it bled. Ted Karalekas, the other owner of the Shipwreck and Leslie’s boyfriend, is a fan of the Vineyard Sound and has taken the singers clamming and fishing. Famous for his Hawaiian shirts, he gave one to Henry, who was determined to rejoin the group, which had headed up to the church, shaken but with a show-must-go-on spirit. As Henry dashed for the church, Ted said, a woman at the Ship Wreck called to him, “Break a leg!”

At the Camp Ground, the Vineyard Sound had gathered behind the Tabernacle. For a few minutes, the singers tried to figure out what might have motivated the push. They knew that some people just don’t like a cappella, but no listener in Vineyard Sound history had ever been driven to violence by it. Henry said he had never seen his attacker before.

Police officers arrived. Henry and the singer who had been standing next to him, Schuylar Rockey, were needed at the station to identify a suspect.

Meanwhile, the assailant had kept ahead of the posse into the Camp Ground, but in pursuit was Zared Shai of Oak Bluffs, who was having dinner at Coop de Ville when he saw the shove. With him were at least two other men. Zared caught the runner in back of Linda Jean’s Restaurant. “He turned to me and said, ‘If you touch me, I will knock you out.’ So I said, ‘Do what you gotta do, man.’ Then he took off running again,” Zared said.

The fugitive ran through the alleyway next to Sharky’s Cantina and onto Circuit avenue, where Zared and one other man cornered him in front of Reliable Market. The runner turned and pushed the second man in the face, breaking his glasses, before taking off up the street again.

It ended when the pusher ran into Kyle Shea, a seasonal officer with the Oak Bluffs police. He tackled the man in the alley beside the Slice of Life restaurant.

Zared and two compadres returned to the harbor in triumph. Ted Karalekis treated the three vigilantes to free beers.

At the church, just before the show, musical director Brendan Dolan quickly rearranged the set list to cover for the two singers who were still at the police station. Three songs into the show, Schuylar and Henry returned. The next song was a fan favorite, and Henry, wearing Ted’s Hawaiian shirt but his khaki shorts still dripping, sang his bass line on Southern Cross.

When the song was over, Eddie told the audience what had happened. “The crowd was really receptive, they were very nice about it, they were clapping and happy that he got caught. We couldn’t have had a better crowd. And we had a little extra energy, because we were so amped up about what had happened.”

Gazette reporter Jim Hickey contributed to this story.