Mysto the Magician appeals to a snobbish opera singer, the Great Poochini (a pun on opera composer Giacomo Puccini), to let him perform an opening act at the show that night. Mysto's tricks primarily come from his magic wand, which can summon flowers and rabbits. After Mysto dances and asks him if he gets the job, Poochini emphatically says "NO!" as he kicks Mysto out the door into the alley.
While on the ground, upsetter Mysto plays with his magic wand, but soon realizes he can pass it off as a conductor's baton, being further inspired by seeing himself in place of the conductor in a promotional poster outside the door and plans to get revenge on Poochini. Later, as the performance is starting he freezes the conductor, steals his tuxedo, nose and hair, then takes his place in front of the orchestra to conduct the Great Poochini, who is unaware of the imposter in front of him.
During the performance, in which Poochini (performed by the Colombian baritone Carlos Julio Ramírez) sings Largo al factotum from Gioacchino Rossini's The Barber of Seville, Mysto unleashes a variety of tricks with his wand. He begins tamely by summoning rabbits and flowers, then turning Poochini into a ballet dancer, Indian, tennis player, prisoner rock-breaker and football player. Mysto's revenge gets more brutal as he throws a cymbal on Poochini's head, turning him Chinese (see below), then transforming him into a country singer and sings, Oh My Darling, Clementine. After levitating Poochini to the ceiling and slamming him down to the stage, Mysto turns him into a square dance caller. Poochini actually continues his performance for a good 20 seconds after this without interruption, except for the "hair gag". Poochini is then transformed into a Shirley Temple–esque child (who sings "A-Tisket, A-Tasket" before the balloon blows up and pops), then a Carmen Miranda–type singer (with two rabbits accompanying him on guitar) after an irritated audience member hurls an armload of fruit onto Poochini's head where it piles up like Miranda's headdress. The same guy later sprays black ink on Poochini turning him into Bill Kenny from the Ink Spots, then he throws an anvil on him, crushing him into a shorter height and deepening his voice as well. After a rabbit hoses off Poochini's face and another rabbit works his arm like an automobile jack to get him back up to full height, the fun continues as he is transformed into a Hawaiian singer with two rabbits for harmony. Reaching the end of the number, Mysto's plan is finally revealed to Poochini as his wig falls off. Mysto quickly puts the wig back on, but it's too late. Now set for revenge of his own, Poochini furiously grabs the hairpiece and puts it on while Mysto tries to flee, but Poochini, having also grabbed the magic wand, stops the magician by using the wand on him as placing Mysto to the stage and unleashes the same gimmicks on the hapless magician at high speed. A red curtain with the words "The End" then falls on the magician and the rabbits (at the end of the Hawaiian singer shtick).
The concept of cartoons with insinuating situations is hardly new—Tex Avery especially featured a few quick jokes of this nature in his cartoons. Magical Maestro, for example, shows Poochini with a male and female rabbit on each arm. He lowers his arms behind his back and when he raises them again, he now has an additional dozen baby rabbits on them, six on each arm.
This cartoon features a gimmick only seen in Tex Avery films, the "hair gag". Because cartoons were shown originally in movie theatres, the film strip, if loaded incorrectly, would rub against the gate mechanism, shaving off tiny "hairs" of celluloid. These hairs would get caught in the 'gate' of the projector. Sometimes it would skitter across the projection light, resulting in a gigantic hair appearing on the movie screen. In this cartoon, the opera singer pauses mid-song to pluck the offending hair from the film and tosses it aside, one of Avery's many ways of his characters breaking the fourth wall. It wasn't the first time Avery used this gag. It was also used in Aviation Vacation (1941).
The role of Poochini is portrayed by Butch the Irish dog, a frequent star of Avery's cartoons of that era (often alongside Droopy).
Since its debut on television, Magical Maestro has frequently been shown with two gags missing, that were edited out due to their reliance on racial stereotypes.Poochini is transformed into a Chinese stereotype when a cymbal is thrown on his head by the conductor, simulating the coolie hat which is wide and flat, with a point in the top. A repeat of this gag near the end of the cartoon (as Mysto is forced to sing and turns into the characters that Poochini was turned to thanks to the magic wand) was also cut.
An irritated audience member in a box seat right above the stage shows his distaste for the performance by spraying ink from a pen into Poochini's face, leaving him looking like a blackface singer singing in the style of The Ink Spots. When this does not shut him up, the audience member drops an anvil on Poochini's head, squashing him down and giving him a deeper voice reminiscent of the Ink Spots' spoken interludes, or of Eddie 'Rochester' Anderson. The edited version shows the audience member about to drop the anvil on Poochini, then immediately cuts to the rabbits jacking him back up to normal height, having already washed the ink off his face.
Magical Maestro was broadcast throughout the 1970s and 1980s in various formats, depending on censoring rules at the particular television station. New York–based WPIX, for instance, edited out the audience member segment, but left the Chinese stereotype in place. Chicago-based WFLD (now FOX-32) as well as Atlanta-based WTBS (the future parent of Cartoon Network), aired the film uncut. The current edited version that has aired on Cartoon Network since the late 1990s is missing both gags.
The "hair gag" would later be used by English comedian Benny Hill in the closing chase sequence of his April 25, 1984 show. As he is being chased by medical staff and an ambulance in and around a hospital area, he notices a hair moving around the bottom right corner of the screen, and at a certain point stops his pursuers long enough for him to pluck the hair out before the chase resumes.
Tom and Jerry Tales episode "Way-Off Broadway" features a gag similar to Poochini's transformations, in that Tom is forced to adapt to various pieces music when Jerry changes them on a radio.