On October 21, 1978, Frederick Valentich departed Moorabbin Airport near Melbourne in a Cessna 182L, registration VH-DSJ, for a night visual flight toward King Island across Bass Strait. Department of Transport records identify Valentich as a 20-year-old private pilot with more than 150 hours of flight experience and a Class Four instrument rating permitting night flight under visual meteorological conditions. VH-DSJ was refueled before departure and left Moorabbin at 6:19 p.m.

At 7:00 p.m., Valentich reported passing Cape Otway and proceeding toward King Island. At 7:06:14 p.m., he asked Melbourne Flight Service whether any known traffic was operating below 5,000 feet. Told that none was known, Valentich began describing what initially appeared to him to be a large aircraft with four bright lights. Over the following minutes he reported repeated high-speed passes, a long shape, a green light, and a shiny or metallic exterior. The official transcript preserves those descriptions but provides no independently measured range, size, bearing, or trajectory for the reported object.

At 7:11:52 p.m., Valentich reported that his engine was running roughly and coughing. Communications continued briefly before his final intelligible transmission at 7:12:28 p.m. The official transcript then records seventeen seconds of open microphone before contact ended. Later accounts frequently describe those final seconds as a metallic scraping or grinding noise, but the official transcript itself does not identify the sound or characterize its source.

Search-and-rescue operations began that evening and expanded on October 22. Aircraft, ships, fishing vessels, police, and coastal searchers examined areas around the intended route and possible drift locations. Search planners did not know the aircraft's final position, and the estimated search datum changed during the operation. Active searching ended on October 25 without locating Valentich, his aircraft, or a body.

Possible debris and an oil slick were investigated during the search. Laboratory analysis of samples from the slick did not positively associate the material with aircraft fuel or lubricating oil. Other reported debris likewise failed to establish the location of VH-DSJ.

A later discovery generated renewed interest in the possibility of wreckage. In 1983, investigators examined a cowl flap found near Flinders Island. The part was compatible with a range of Cessna 182 aircraft that included VH-DSJ, and investigators examined whether ocean currents might have transported it from the Cape Otway–King Island area. The surviving correspondence does not uniquely identify the component as belonging to Valentich's aircraft.

Investigators considered several possible explanations for the disappearance. These included spatial disorientation followed by impact with the sea, an emergency connected with the reported engine roughness, a controlled water landing, diversion or landing elsewhere, deliberate disappearance, and the possibility that the unidentified phenomenon reported by Valentich was involved. A Department of Transport human-factors review considered disorientation a serious possibility but did not establish it as the cause.

Astronomical and environmental factors also became part of later discussion. The National Archives of Australia notes that bright celestial objects were visible that evening and has discussed the possibility that Valentich misinterpreted astronomical lights while becoming spatially disoriented. The official investigation itself, however, did not establish a particular celestial source for the observations.

Radar records did not provide an independent track corresponding to the object Valentich described. The surviving aviation file documents anomalous propagation and uncertain radar returns, but later technical review interpreted those returns as consistent with propagation effects rather than a verified aircraft pursuing or circling VH-DSJ.

The Department of Transport investigation eventually retained an undetermined conclusion. Its published summary states that the location and time of the occurrence were unknown and that the reason for the aircraft's disappearance could not be determined.