Gyro Gearloose tests on Donald his latest invention, a special water that makes invulnerable whoever is soaked in it. When Donald leaves, Gyro thinks there's a thing about the water he forgot to tell him, but doesn't remember what.
Back at home, Donald doesn't find Huey, Dewey and Louie, but they have left a note telling uncle Scrooge wants to see him. Donald goes to Scrooge's home just to have the pleasure to deny his help. There, Scrooge explains his year earnings reach two thousand millions one dollars, and that single extra dollar may mean one additional million in Scrooge's taxes, so Scrooge wants Donald to impersonate the Duck Avenger and steal twenty thousand million dollars from him, just to return them after Scrooge pays his taxes without the additional million. Donald refuses to stain the Duck Avenger's name, as it would be the same if the thief is the Avenger or himself, which Huey, Dewey and Louie see as a confession of Donald being the Avenger, but Donald denies it and tries to leave, but Scrooge tries to shoot him and Donald uses the fake Duck Avenger cape as a shield, so it gets ripped.
While Scrooge patches the cape and thinks who else could do the impersonation, Donald also thinks Scrooge will hire someone else, so the real Duck Avenger will have to stop him. Donald is confident as he is invulnerable now, but ther's still something Gyro hasn't told him about the invulnerability water.
Donald pretends to be tired, but as his nephews suspect his double identity, they also pretend to go to bed and prepare a trap with marbles that will make him trip towards the stairs and on a mattress (as they want to discover him, not harm him).
Meanwhile, Gyro awakens as he finally remembers what he had to say: the invulnerability water will lose its effect at midnight. He tries to call at Donald's house but Donald and the kids fall on the trap and before they reach the phone, Gyro hangs believing they all are asleep, so he'll call tomorrow. Donald has the kids retire the matress and gives them some milk with sleeping pills. While they discover it, they have already drank enough to fall asleep.
Meanwhile, Scrooge has patched the torn cape and at 23:00 both the real and the fake Duck Avengers go to the bank. When the guard finds both, the actual Duck Avenger proves his identity by having the guard shoot him, and he's unarmed. The shooting noise awakens the neighbors who form an angry crowd, so the guard and both Duck Avengers run away.
The real Duck Avenger hides until the crowd leaves and then jumps, but it's midnight and he hurts himself with some rose thorns, so he finds he isn't invulnerable anymore. The Duck Avenger quickly runs back home and stumbles with a trashcan where the pretender was hiding. Then Donald and Scrooge recognise each other's voice, so the Duck Avenger runs home to change into Donald. Scrooge follows him but when he enters Donald's house, he trips on the marbles left from Huey, Dewey and Louie's trap.
The kids awake and Donald leaves his room in pajamas, as an alibi to his identity. they find Scrooge disguised as the Duck Avenger, but recognise he's not the real one as he has the patched cape. Donald unmasks Scrooge, who has broken his leg and is happy for it as the tax will be reduced anyways due to his medical bill. The kids tell uncle Scrooge is lucky, but Donald reminds them this was all thanks to the Duck Avenger.