Mrs. Rittenhouse, a prominent socialite, hosts a lavish weekend party at her Long Island estate to celebrate the return of the esteemed African explorer, Captain Jeffrey T. Spaulding. The highlight of the evening centers around an art reveal, featuring the newly acquired painting "After The Hunt" by the artist Beaugard, with musical accompaniment from Signor Emanuel Ravelli and his enigmatic partner, known simply as the Professor.
Intriguingly, Captain Spaulding makes a flamboyant entrance in a sedan chair, but shortly after arriving, he announces his plan to leave by performing the song "Hello, I Must Be Going." Meanwhile, Mrs. Rittenhouse's daughter, Arabella, hatches a scheme to further her fiancé, John Parker's, career in the arts. John has produced a nearly flawless replica of the Beaugard painting during his studies, and Arabella believes that switching his copy with the original could impress Roscoe W. Chandler, the art patron responsible for acquiring the work. She recruits Ravelli to assist in this ambitious plan.
However, unbeknownst to Arabella, two other guests, Grace Carpenter and Mrs. Whitehead, have devised a similar strategy—though their motivation is to humiliate Mrs. Rittenhouse rather than to advance anyone's artistic pursuits. Grace has crafted a subpar imitation of the painting, and enlists the help of Hives, the butler, who previously worked for Mrs. Whitehead, to facilitate the switch. In a twist of fate, they inadvertently replace John's superior copy instead of the original painting.
As tensions rise, the plot thickens when Ravelli and the Professor realize that Chandler is none other than "Abie," a former fish vendor from Czechoslovakia. In a chaotic scene during a thunderstorm that darkens the mansion, the two attempt to orchestrate their own painting switch when Captain Spaulding and Mrs. Rittenhouse interrupt their efforts.
During the climactic unveiling of the painting, Chandler quickly spots the inferior quality of Grace's replica, which leads him to the realization that his coveted artwork has been stolen. John, oblivious to the true situation, mistakenly believes his own work remains on display and feels disheartened by Chandler's criticism. A second power outage further complicates matters, resulting in the disappearance of even the substitute painting.
The following day, the police arrive to delve into the art theft. John locates Grace's inferior version and, alongside Arabella, begins to understand that multiple schemes have been at play. After retrieving his replica from the Professor, John and Arabella present their discoveries to Captain Spaulding. The Professor is detained, and all three paintings are ultimately recovered and identified. As a surprise twist, when Chandler mistakes John's skillful copy for the genuine Beaugard artwork, he recognizes John's talent and commissions him for a series of portrait paintings.
In a humorous twist to conclude the film, the Professor manages to escape the authorities and takes it upon himself to use a Flit gun to spray the assembled guests with a sedative, rendering them all unconscious. Ultimately, he too succumbs to the effects of the substance, leaving all the characters sprawled out on the floor in a comical end to the chaotic events.
