On the day of Fadinard's wedding, his horse eats the eponymous hat on a bush at the roadside, while its owner is hidden behind the bush with her lover. Because she is married, she cannot return home hatless without being compromised.
Determined to avoid suspicion by procuring a replacement, the hat's owner and her lover follow the hapless groom back home and charge him with finding a similar hat. In the ensuing frantic search Fadinard embarks on a whirlwind tour of a variety of locations, accompanied by eight taxi-loads of wedding guests.
The plot thickens - it is more accurate to say it stretches - to encompass a cast of characters including an imperious father-in-law, a milliner, ex-mistress of Fadinard, a high-society Baroness, the cuckolded husband, a fantastically flamboyant Italian composer, an idiot cousin besotted with the bride and a doddering uncle, all conspiring to foil Fadinard's attempts to keep up appearances in the face of chaos.