A quiet summer night, while Duckburg sleeps, the Duck Avenger patrols the city. He sees a robbery but before he can act, a new superhero named "Mister X" scares the thieves, and fancies himselfthe Duck Avenger's competitor. The Avenger continues his patrol and finds two thugs breaking into a store, but Mister X gets ahead of him again and the thugs are scared by the cops' arrival. Later, the Beagle Boys are about to steal Scrooge McDuck's Money Bin, so the Duck Avenger tries to scare them with a flashlight, but Mr X does it first with a huge light. Scrooge awakens to see the Beagles running away and Mr X presents himself to him.
At dawn, the Duck Avenger returns home and, as Donald, tries to have a relaxed breakfast, but he gets irritated by the radio praising Mister X and accidentally ends with the breakfast on his face. He decides to go to sleep til his next night patrol.
That night, Mister X gets ahead of the Duck Avenger helping a man who lost his keys to get into his house, and repairing a general city blackout. A sad Duck Avenger returns home and goes to sleep, but Donald is awaken by a call of Uncle Scrooge asking him to tell the Avenger he will no longer be needed, as Mister X will help Scrooge with an important gold cargo. Mister X also gets something the Duck Avenger never had: a live interview.
Donald watches the interview where Mister X gets offended at the idea of following the Duck Avenger's steps, replying he is much better. The interview ends with the mayor giving Mister X something else the Duck Avenger never had: an official Duckburg Superhero title. The mayor also declares the Duck Avenger can still act as Mister X's substitute, a vicesuperhero. Donald gets angry and decides to leave his double identity and destroy his Duck Avenger gadgets. Before he can do it, though, Huey, Dewey and Louie return from Grandma Duck's farm.
While Donald and the kids eat some cake sent by Grandma, they talk about Mister X, Duckburg's new hero. Donald is happy to know the kids prefer the Duck Avenger, as Mister X is smug, talks badly about the Duck Avenger, seemingly appears before the crimes happen and never catches the villains, just makes them run away. This makes Donald suspicious.
That night, the Duck Avenger goes to watch Scrooge's gold cargo that will be escorted by the Duck Avenger. The Avenger puts a transmitter on the truck and follows it with the 313-X. The transmitter allows the Avenger to hear when Mister X sprays sleeping gas on Scrooge and the truck driver, and then gloat to himself that it was easy to get ahead of the Duck Avenger as he already knew where his accomplices would be. Mister X reunites with the Beagle Boys and takes away his superhero costume, as he was a Beagle in disguise. When the Beagle Boys enter the truck trailer to see the gold, the Duck Avenger surprises them and closes the door, leaving them locked.
Later, the Duck Avenger is being interviewed, and the reporter asks him when did he begin suspecting Mister X. The Avenger tells it was an intuition to tell other day, and he actually wants to adress some people who quickly disqualified him without considering his old merits (specially the mayor and Scrooge), and asks if they still want him in Duckburg. Donald's nephews watch the interview from their house and say definitely yes.
- While Grandpa Beagle appears at moments in the story, his beard is unpainted and looks like his grandkids' stubble, being only recognizable by his corn pipe.