Mickey Mouse, Donald Duck, Goofy, and Mickey’s dog Pluto are street urchins, who, while being robbed by masked bandits (played by the Beagle Boys) are saved by the Royal Musketeers, Athos, Aramis, Porthos and D’Artagnan. Mickey is gifted one of their hats, inspiring him to follow their example and become a musketeer; however, in the present day, he, Donald and Goofy are janitors for the musketeers’ base.
Meanwhile, Minnie Mouse, princess of France, and her lady-in-waiting, Daisy Duck, are in a palace discussing Minnie’s obsession with finding her ”one true love”. Daisy says that she must marry someone who is of royal blood, and Minnie insists that she cannot marry someone she does not love. Minnie says she will know that he is ”the one” when he makes her laugh. Minnie then takes a walk in the palace garden and barely escapes with her life when the Beagle Boys attempt to drop a safe on her.
The Beagles run to tell their boss, Captain Pete, that they were not successful in dropping the safe on Minnie; he clobbers them, saying he wanted her kidnapped until he can take over the kingdom. He then went to the janitor room, telling Mickey, Donald and Goofy that they passed his test and have what it takes to become musketeers.
While Minnie and Daisy, protected by Mickey, Donald, and Goofy, go on a journey, they are ambushed by the Beagle Boys. Donald hides and is eventually thrown off the carriage, and Goofy is easily defeated, leaving Mickey to fight the intruders. Mickey is also defeated, leaving the three heroes stranded. Mickey encourages his friends not to lose hope and they rush to rescue Minnie and Daisy.
Pete is furious that the Beagle Boys failed in their task and realizes that the three protagonists are more of a threat than he originally anticipated. He then plans to get rid of them one by one. While on night duty, Goofy is lured away from the palace by Clarabelle (who uses a shadow puppet of Mickey). The Beagle Boys appear before Donald, capture him, and Pete tries to do away with him with a guillotine, but he escapes at the last second. Pete tries to go after him by foolishly going through the guillotine’s hole with the blade still coming down and winds up chopping off his peg leg. Donald returns to the castle and tells the whole story to Mickey (who doesn’t believe him at first) before running off, leaving Mickey by himself. Mickey is then captured by Pete, who chains him up in a dungeon in Mont Saint-Michel that will flood when the tide comes in. The trio eventually escape and reconcile with each other and set off to rescue Princess Minnie.
Minnie is captured by the Beagle Boys and the smallest one poses as her, announcing to the public that she is handing the crown over to a gleeful Pete and has a false advertisement that says ”King Pete the Magnificent”. Mickey, Donald and Goofy arrive and battle Pete and the Beagle Boys onstage, finally defeating them and saving the Princess (though what became of the villains is unknown). Mickey and Minnie finally declare their love for one another, as do the others (Donald and Daisy, Goofy and Clarabelle). Mickey, Donald and Goofy are dubbed royal musketeers.