When wealthy patron of the arts James Spragg decides to give away a genuine valuable Stradivarius violin to the winner of a competition at Vocalion Hall, local mobster Halloran hatches a scheme to get it. His goons kidnap this local "spoiled brat" violinist while he's riding his pony in the park alone. They disguise him as a poor kid, and threaten him into winning the contest. Cosgrove wins with a special arrangement of Mendelssohn's "A Midsummer's Night Dream," Halloran grabs the Strad, and superhero the Black Terror gives chase.
After the Terror Twins defeat the mobsters, Cosgrove nobly turns down the violin, saying a really poor boy should have it.