Rock opera is not a term used a lot today. Maybe it never has been.

It came into the public consciousness in the late 1960s, with “Tommy” by The Who as probably the most famous example. Other than that, can you even name another rock opera?

That’s what makes the current tour by progressive metal band Dream Theater so unusual. The band is bringing the music from its brand new rock opera album, “The Astonishing,” to theaters around the world. The show will come to Portland’s Merrill Auditorium on Oct. 21.

A rock opera is generally considered a collection of rock songs connected by a story. But it’s not scripted for actors, that would make it a rock musical. So when Dream Theater performs “The Astonishing” in Portland, audiences will hear two hours of music that tell the story of a dystopian future America, about 300 years from now.

Dream Theater has been together, with various members coming and going, for some 30 years. For this tour, they decided to leave their extensive song catalog at home and just do “The Astonishing.”

“We wanted to do this in small theaters with good acoustics, because we felt like we had something unique to share,” said Jordan Rudess, keyboard player and co-writer of the music for “The Astonishing.” “The music is meant to support the story, and it’s turned into the biggest production we’ve ever done.”

The story and lyrics were written by guitarist and original member John Petrucci. The story follows a rebel militia in its efforts to defy “The Great Northern Empire of the Americas” by using the power of music. The society is basically run by machines, with the aid of a collaborating governing body, Rudess said. The hero of the story is a man who has the gift of music and uses it “to make a difference in his society.”

“It looks at technology, at the idea that, if we’re not careful, things can go in a very different direction,” Rudess said.

Besides the dramatic, hard-rocking music, the performance will include video images and lights that help tell the story. The band hired a production company to help stage the show, Rudess said.

“We want this to be an immersive experience for people, the minute they walk through the door,” he said.