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When bumbling sailor Tim Dunnovan receives word that his aunt has died and left him $1,450, he stuns his best friend, the conniving Benny Linn, by buying a racehorse.  Tim and Benny's harried superior officer assumes that Benny was swindled and gives both sailors five days of leave to straighten out the matter.  At the stables where Tim's new horse, Little Erin, is kept, Tim and Benny confront "Doc" Garvey, the horse's trainer.  Although Garvey defends the sale, jockey Skeezer sneers at the horse and his weak ankles.  Little Erin's devoted young groom, The Pearl, insists that the animal's ankles will improve with rest and reveals that the horse has a healthy twin brother, Little Shamrock.

Sensing exploitation possibilities in twin horses, the sailors take Little Erin to the drive-in restaurant where Little Shamrock's owner, Jane Sweet, works as a carhop.  The dimwitted Jane confirms that the two animals are identical and takes Benny and Tim to visit Little Shamrock, a farm plough horse.  Benny schemes to switch Little Erin and Little Shamrock and trick Little Erin's previous owner, Bert Sedgwick, into buying him back.  Posing as Southern gentlemen, Benny and Tim drop by Bert's office and inform him that Garvey swindled him by passing off Little Erin as lame.

Bert, whose socialite fiancée, Millicent Temple, has forced him to give up his horseracing interests and become a businessman, sees through the sailors' impersonation, but when Benny insists that Little Erin is racing well, he agrees to take a look at the horse.  At the stables, Skeezer races Little Shamrock around the track, impressing Bert, who thinks the horse is Little Erin, with his time.  Before Bert can make Tim an offer, however, Millicent shows up and warns Bert not to get involved.

Determined to change Bert's mind, Benny enters Little Shamrock in a race and plants a newspaper story about the twin horses.  After reading the story, Bert concludes that the sailors are trying to trick him and rushes to the track.  There, Benny, Garvey and Skeezer all slip Little Shamrock vitamin pills, and the charged-up horse wins the race.  Assuming that Little Erin is really Little Shamrock, Bert offers Benny and Tim $5,000 for Little Erin, but says that, because of Millicent, his name cannot appear on the ownership title.

Later, Bert, who is attracted to Jane but reluctant to break with Millicent, invites her for a drive, and the two kiss.  After Bert takes Little Erin away, Skeezer reveals to the sailors that The Pearl switched the horses yet again.  Concerned because their names are on the real Little Erin's title, Benny and Tim rush to Bert's office.  There, Bert informs Tim and Benny that he is entering Little Erin in a $100,000 race, and if he wins, he will give them a cut and officially assume titleship.  Aware of Bert's plan, Millicent tells Tim and Benny that if Little Shamrock wins, her uncle, Admiral Temple, will have them kicked out of the Navy.  To avoid dismissal, the sailors decide to enter the real Little Erin in the race, but their plan goes awry when gangster "High Life" informs them that he now owns half of Little Shamrock and threatens to kill them if the horse does not win.  Later, on their ship, Benny gets the idea to kidnap both horses and sneaks the animals to the ship's brig.

The following day, Bert sees a newspaper report about the kidnapping, as do Jane and High Life.  That night, at the end of a friendly date with Tim and Benny, Jane confesses that she is sad because now that the horses have been kidnapped, her chances of winning Bert away from Millicent are nil.  For Jane's sake, Tim and Benny decide to run Little Shamrock in the race, but as they cannot tell one horse from the other, remove both from the ship.  As they are sneaking off, Tim and Benny knock out two civilians they assume are High Life's goons and toss their ticking suitcase overboard, setting off an explosion.

The next day, at the racetrack, Garvey, who has bet on High Life's other horse, offers Skeezer half of his winnings not to race Little Shamrock.  Skeezer agrees, and Benny must fill in as jockey at the last minute.  When Jane then dashes up with the news that Little Shamrock is really Little Erin, Tim mounts Little Shamrock, and both horses run the race.  After a healed Little Erin finishes first with his brother, Millicent gives the jubilant Bert an ultimatum.  To her horror, Bert chooses horseracing, and Jane.  Both horses then are disqualified, and High Life's goons come for Tim and Benny.  Before either the gangsters or the sailors' equally angry shipmates, who also bet on Little Shamrock, can pummel them, however, Tim and Benny are ordered back to their ship.  Instead of the expected punishment, the two are awarded medals for their bravery in apprehending the two saboteurs.