A despondent Captain Zaharie Ahmad Shah, troubled over the end of his marriage or some other unbearable sadness, was the only person awake on the doomed plane full of unconscious people, one theory goes.
The suicidal pilot disabled communications on Malaysia Airlines Flight MH370, took a final pass over his hometown, then torpedoed the Boeing 777 into the Indian Ocean, experts insisted.
A competing theory, the official one posited by investigators, says that everybody on the plane - the captain, his co-pilot, more than 200 passengers - was unconscious as the uncontrolled craft ran out of fuel and plunged into the sea.
The disappearance of MH370 was a tragedy, government investigators say - but an accidental one.
Four years after MH370 vanished, the debate over what, exactly, happened to the plane has reignited.
The back-and-forth comes after a panel of aviation experts assembled by the Australian edition of 60 Minutes posited a new theory about the final hours of the flight.
The man in charge of the Australian Government's investigation has struck back, saying the "new" theory has holes of its own.
That theory is that Zaharie, a veteran pilot, depressurised the plane after turning off its transponder.
Shortly afterwards, everyone else on the plane was knocked out by oxygen deprivation, unwittingly being ferried to their deaths.
The reason for Zaharie's supposed suicidal ideation? There were rumours that his marriage was ending and his wife was about to leave him.
"He was killing himself," Larry Vance, a veteran aircraft investigator from Canada, said on the 60 Minutes panel. "Unfortunately, he was killing everyone else onboard. And he did it deliberately."
The 60 Minutes experts' theory attempted to answer one of the biggest questions surrounding the ill-fated flight: How could a modern aircraft tracked by radar and satellites simply disappear?
Because, they say, Zaharie wanted it to. And the pilot, who had nearly 20,000 hours of flight experience and had built a flight simulator in his home, knew exactly how to do it.
Shortly after the 60 Minutes episode aired in Australia, critics of the Australian Transport Safety Bureau's investigation began to speak out.
Clinging to the MH370 accident theory was akin to "complicity of a crime," Mike Keane, a former military pilot and the former chief pilot at Britain-based easyJet, told the Australian.
"Put bluntly, the MH370 'crash' is undoubtedly a crime of the unlawful killing of 238 innocent people. The Australian Government has also been remiss, they should have put pressure on the ATSB to listen, and act, on professional advice from the aviation community."
But people on the other side of the debate have said there are holes in the suicide-by-pilot theory, including the basic biological difficulty of flying a depressurised plane.
"What they fail to understand is that while you don an oxygen mask and prevent the worst of the hypoxia situation, you are flying an aircraft at 40,000 feet," the ATSB's Peter Foley told lawmakers at a hearing in Canberra yesterday, according to the Guardian.
Foley, who led the ATSB's failed search for MH370, continued: "You are taking an aircraft from sea level to [Australia's highest mountain] Mount Kosciuszko in 20 minutes, then you are taking it, over the course of a couple of minutes, to the height of Mount Everest plus 1000 feet. You'll get decompression sickness too."
He said a similar situation occurred on a cargo plane in the United States nearly 25 years ago.
"During the climbout the flight crew was unable to pressurise the aircraft, and the captain elected to proceed with the flight," Foley said, according to the Guardian.
"The crew donned their oxygen masks and shortly thereafter the captain became incapacitated from decompression sickness. The first officer took command and they landed the plane."
Still, Foley conceded to the Australian legislators yesterday that the MH370 suicide-by-pilot theory was "plausible" and that the ATSB had listened to experts who supported the "controlled ditching" theory.
And the death-by-pilot theory adherents have an answer for something that has vexed investigators for years: a circumstantial but chilling explanation for an unexpected turn the plane made while passing over Zaharie's Malaysian hometown.
"Captain Zaharie dipped his wing to see Penang, his hometown," Simon Hardy, a Boeing 777 senior pilot and instructor, said on 60 Minutes.
"If you look very carefully, you can see it's actually a turn to the left, and then start a long turn to the right. And then [he does] another left turn. So I spent a long time thinking about what this could be, what technical reason is there for this, and, after two months, three months thinking about this, I finally got the answer: Someone was looking out the window."
"It might be a long, emotional goodbye," Hardy added. "Or a short, emotional goodbye to his hometown."