Milo goes to the opera and when things go wrong, he ends up center stage. 
Milo, Zack, and Melissa attend an opera to receive extra credit for Ms. White's class, running into fellow classmate Amanda Lopez on the way. She admits that Milo's presence at the show makes her nervous, as she loves opera for its consistency and order; something which could be easily ruined by Murphy's Law. After assuring her that things would be fine, Milo leaves to buy some pistachios at the concession stand, only to have thevendors refuse to sell. He wears them down until they agree, but it quickly becomes obvious that they don't have any experience with the dispenser, and the resulting flood of pistachios sends Milo to the basement.
The show begins, and Zack leaves to find Milo, who'd ended up backstage in an attempt to prevent anything from going wrong. Melissa, meanwhile, does her best to keep Amanda calm despite the increasing chaos.
The clear danger on set causes one of the main actors to walk out while putting Amanda on the verge of a panic attack. Seeing this, Milo quickly takes the actor's place and the show goes on. Everyone in the audience gives a standing ovation and the four go for pizza.
For a full transcript of "Smooth Opera-tor", click here.
They are seen separately afterwards among many other less notable Phineas and Ferb Background characters
Among the many objects that fall from the roof when the background screen collapses are the cat, the two elders playing checkers and the guy with a poodle who were also disturbed when the football field was ripped up by a drill in Rooting for the Enemy
The performers who join together for the impromptu finale includes all 6 actors (7 including Milo) seen on stage, two actors who were startled by the fire extinguisher backstage, and the man in the giraffe costume who ran across the stage earlier, leaving only two people previously unseen in the episode.
"Smooth Opera-tor" was one of the first episodes of the series to premiere on Disney Channel before airing on Disney XD.
During the performance, the positions of the audience around Melissa and Amanda changes. Most notably, in some shots, Milo and Zack's vacant seats have been taken