Dispatch drummer Brad Corrigan and guitarists Pete Heimbold and Chad Urmston played at the Somerville Theater last night.

Fans of the wildly popular band Dispatch had better be ready. It’s all coming down in one fell swoop.

Brad Corrigan, Pete Heimbold and Chad Urmston will reunite after two years apart for one final concert tomorrow afternoon at 5 p.m. at the Hatch Shell in Boston. The performance, open to the public and free of charge, is expected to draw an audience more than 50,000 strong, including listeners from as far away as Australia and New Zealand, according to submissions to the band’s official website.

“We don’t really buy the authenticity of our musicianship and our playing,” Corrigan said. “We always see there’s a lot of room to grow and improve but people keep showing up. And then to find out that there are 15 different countries that are going to be represented at the show on Saturday? Never.”

Nearly a decade ago, thoughts of an international legion of devotees and a sprawling hometown finale hadn’t even begun to materialize. The name “Dispatch” hadn’t yet entered the American musical lexicon. But, tucked away at Middlebury College, “three hackers who used to play sports and had guitars in their hands,” according to Corrigan, had only begun to appreciate their combined talent, then performing under the name “One Fell Swoop.”

But, after ceding the rights to that name to a St. Louis band—“a little country act, punks,” according to Corrigan—the group adopted the name by which it is currently known after shuffling its instrumental organization, with Corrigan shifting from guitar to drums, thereby paving the road that finally winds to its cul-de-sac tomorrow.

“It just gave us all the flexibility that we wanted,” Corrigan said. “We probably looked for guys to play with for maybe six months, a year, and just didn’t seem right to add players, and we were kind of getting burned out on guitars...It’s like going from black and white to color [and] we could do anything we wanted to do.”

“Put a boxing ring in the middle of the stage and put Bob Marley in there with a couple of the guys from Pearl Jam and Paul Simon,” Corrigan said of the band’s influences. “Even though he’s a little guy I think he’d had have to get in the ring, and I think you’d have to throw in some of the classics too—some of the guys from Zeppelin. It’d probably turn into a wrestling match, tag team, with Zeppelin, CCR and Crosby, Stills and Nash, and then I’d add Anthony Kiedis from the Chili Peppers—I don’t know. We’re a mutt. We’re not a pure blood band.”

And, due to the unforeseen help of the internet, their efforts did not go unnoticed.

Though the band received minimal radio play while doing gigs in Boston and New York, the trio developed a dedicated underground following among teenagers thanks to the emergence of filesharing programs like Napster.

“We went out to California and we had 1,200 kids there. And they all sang the songs and it was our first time there,” Corrigan said. “What just happened? We talked to a bunch of kids and immediately our eyes were [opened].”

Despite the fan base cultivated by the new medium, person-to-person sharing quickly emerged as a threat to the music it popularized.

“A person came up to me and said, ‘I’ve been listening to you through Napster and I haven’t bought a record in four years,’” Corrigan said. “I said to him, ‘You don’t get it. You’re killing us. Use it to expose music, turn your friends on to it. You’ve found something, now bust your ass to support it.’”

Previously consumers appreciated the value of the work needed to produce an album, but the ease in procuring songs illegally now has almost entirely eliminated consideration of the artist, according to Corrigan.

“People think that stuff with soul is free, but they’ll buy water for a buck fifty at a 7-11,” Corrigan said. “They won’t pay for a song. Are you kidding me?”

Mustering up that emotional energy proved increasingly difficult in 2001, when the band hit a creative wall, burned out from the strain of their schedule. Longing to spend time with family and friends, the trio’s relationship began to unravel offstage.

“Our friendships were in the toilet between the three of us even though our success was looking up,” Corrigan said. “There was just no amount of money or press or people telling us we should stick it out that was enough for us to be happy.”

“It’s just that our country tells us to ride stuff into the ground, kill it, milk it for every dollar,” Corrigan added. “And that’s lame. You’ve got to know when to let go of something.”

So in 2002, Corrigan moved back to Colorado, temporarily suspending the schedule of one of the college circuit’s most popular bands.

Now, two years later, the music has, for one last time, called their name out loud.

“We just figured, you know, let’s get together, see how we feel together, get all of our crew and family, friends, get everyone in on it again and celebrate it and then just let it go,” Corrigan said. “If it’s meant to come back to us it will.”

Although Corrigan was not yet sure what song the band will close its final encore with, he said that he hopes for one in particular: “The General,” the immensely popular ballad about an army officer who turns his back on and saves his men from needless conflict.

Who is the general, though?

“Don’t we all want to know?” Corrigan asked.

—Gregory B. Michnikov contributed to the reporting of this story.

—Staff writer Timothy J. McGinn can be reached at mcginn@fas.harvard.edu.