An outbreak of acute gastroenteritis (AGE) occurred in Breuil-le-Sec, a town in the Oise department (60), in April 2019, coinciding with a report received by the Hauts-de-France Regional Health Agency (ARS) Hauts-de-France, of a contamination alert in the drinking water network caused by a sewage backflow at the wastewater treatment plant (WWTP) in that same municipality. Given the scale of the outbreak and its impact on the local population, an epidemiological investigation was conducted by the Hauts-de-France Regional Unit of Santé publique France in May 2019 among all residents of the municipality. The objectives were to describe the scope and characteristics of the outbreak, to confirm the primary hypothesis of waterborne contamination, and to verify whether a dose-response relationship existed between water consumption levels and health impacts. The epidemiological investigation revealed that the GEA outbreak lasted approximately 10 days, with a peak on April 26. The shape of the observed epidemic curve pointed to a common, point source of contamination, which is consistent with the incident of wastewater backflow into the drinking water system that occurred at the wastewater treatment plant on April 23, 2019. The water use restrictions implemented by the ARS throughout the municipality on April 29 led to a significant decrease in the number of new cases observed in the days following these measures. In addition, a microbiological investigation revealed the presence of a bacterium from the Campylobacter family in some of the sick individuals who underwent stool testing. This GEA outbreak, which occurred in April–May 2019, had a profound impact on the residents of Breuil-le-Sec due to its scale, severity, and the crisis situation it generated. This event should help support awareness-raising efforts among stakeholders (operators, healthcare professionals, local government officials) regarding reporting, as recommended in the guide for investigating outbreaks linked to the consumption of tap water. A more comprehensive and detailed report, presenting, in addition to this epidemiological investigation, the results of the environmental and microbiological investigations, is currently being drafted by the Hauts-de-France Regional Unit, in collaboration with the Hauts-de-France Regional Health Agency (ARS).