Because entrepreneur Peter Randolph, owner of Randolph Motors, needs money to back his new diesel engine, he orders his daughter Patricia to arrange to have wealthy Bostonian stockholder Aloysius P. Merriweather come to dinner at their Bayside estate. Aloysius, a charming man-about-town known for his pranks, sends his barber, Joe Jenkins, in his place, calling him "Mr. Cinderella." Meanwhile, Aloysius, goes out to meet his girlfriend Mazie and is hit by a car and taken away in an ambulance, unidentified. At the dinner party, Joe is assumed to be Aloysius, and bumbles his way into high society. He is much taken by the beautiful Patricia. While her father is detained, Patricia takes Joe on a motor boat ride, which is a fiasco. After running out of gas, getting soaked, and setting Randolph's island cabin ablaze, Joe and Pat spend the night in the sand. In the morning, Joe reveals his true identity to Pat, who convinces him to stay Aloysius in order to prevent a scandal over their having spent the night together. Society gossip spreads quickly about the Randolph-Merriweather tryst and Pat's aunt, Penelope Winfield, calls a newspaper's society editor to announce an official engagement. Randolph, believing that Pat was merely carrying out her father's orders to keep Aloysius on the estate, gets down to business, hoping to convince Aloysius to lend him five million dollars. While Randolph is talking with Aloysius, Joe loses his trunks while swimming and comes upon the yacht of oilman Emmett Fawcett, of Fawcett Gasoline, who is spying on Randolph to see if he really has a diesel engine. Meanwhile, Mazie pretends to be Aloysius's wife, and her brother, Spike Nolan, vows to go to Bayside and kill Aloysius. At the same time, Mazie's friend Lil tips off the police to Aloysius's impending murder. Aunt Penelope arrives at Bayside and begins making wedding plans, until Mazie enters as Aloysius' wife and Penelope faints. Detective McNutt then arrives to prevent Aloysius' murder. Randolph confesses that he's on the verge of bankrupcy and that all Penelope's money is in it, too. When Spike arrives, Pat and Joe kiss while hiding in the attic, planning to sneak into town to find Aloysius. Aloysius, meanwhile, wakes up in the hospital at midnight and, after going to his hotel, discovers all, and points to Joe to save his life. A chase ensues and, when Spike runs out of bullets, McNutt arrests him. Joe and Aloysius finally reveal their true identities and Aloysius tells Randolph that he has no money except what is tied up in Randolph Motors. Joe then comes up with the idea of getting the money from Fawcett by peddling the body of one of Randolph's diesel models without an engine past Fawcett. As the Randolph's butler Watkins peddles, Joe drives, and Fawcett offers Randolph five million dollars to keep the car off the market. As Fawcett pays Joe, Pat appoints him Randolph Motors' new vice president. He then asks Watkins to be his Best Man.