Russian opposition leader Alexei Navalny is in a coma in a Berlin hospital, and Germany has revealed he was poisoned by a Novichok nerve agent.
He was taken ill on board a return flight from Siberia to Moscow and the plane made an emergency landing in Omsk. Two days later Russian officials were persuaded to let him be airlifted to Germany.
BBC Russian has pieced together the story of how flight attendants and medics fought to save his life over the skies of Siberia. This is the dramatic two-hour timeline of that perilous journey.
How the morning unfolded
It was 20 August, and Alexei Navalny was taking an S7 airlines flight from Tomsk to Moscow. He didn't eat or drink anything all morning - apart from a cup of tea he bought at Tomsk Bogashevo airport, according to his press secretary Kira Yarmysh.
Another passenger on the flight, Ilya Ageev, saw Mr Navalny drinking the tea about an hour before the plane was due to take off. The Kremlin critic was smiling and joking with fellow passengers who recognised him.
During the first half hour of the flight, Mr Navalny started to feel unwell. Flight attendants were handing water out to passengers, but he turned it down. He then got up to go to the toilet.
Another passenger tried to use the toilet at the same time, but Alexei Navalny was inside for about 20 minutes. A queue began to form outside the door.
By now all four flight attendants on board were aware one of their passengers was unwell.
Minutes later, a flight attendant made an announcement asking if any doctors were on board. The other passengers now realised the situation was serious.
The rest of the cabin crew informed the pilot and tried to administer first aid to Mr Navalny.
His assistant, Ilya Pakhomov, walked down the aisle appealing for medical assistance. A woman, who hasn't been identified, came forward to say she was a nurse.
For the next hour she and the flight attendants focused on keeping Mr Navalny conscious until the pilot could make an emergency landing, according to S7 airlines.
'He wasn't speaking - he was just screaming'
Sergey Nezhenets, a lawyer, was sitting in the back row close to where Mr Navalny was being treated. He was due to transfer in Moscow before flying on to Krasnodar in southern Russia.
"I started paying attention to what was going on when a flight attendant asked for medical professionals on board to come forward," Mr Nezhenets told the BBC.
"A few minutes later, the pilot announced we would be landing in Omsk, because a passenger was unwell. I only realised the passenger in question was Navalny after we landed, when I checked Twitter and saw his spokeswoman's posts.
"A few minutes after the call-out for a doctor, Alexei started moaning and screaming. He was clearly in pain. He was lying on the floor in the part of the plane reserved for cabin crew. He wasn't saying any words - he was just screaming."
That was when a nurse went forward to offer medical assistance, he explains.
"I don't know what they were doing, I didn't see," he says. "But I heard them keep on saying 'Alexei, drink, drink, Alexei, breathe!'
"When he was moaning, the rest of us felt better, in a way because we could tell he was at least still alive. I stress, at that point I didn't know it was Navalny."
Two of Mr Navalny's assistants were standing nearby; one was his press secretary Kira Yarmysh.
"She was very nervous," Mr Nezhenets says. "The medic asked her what had happened to him, and Kira said: 'I don't know, he was probably poisoned'."
The crew moved fast to ask permission for an emergency landing at Omsk, the airline says, and it was given immediately.
It took little more than 30 minutes for the plane to land after passengers were told there would be an emergency landing.
But the cabin crew "kept checking the windows and complaining that, because it was so cloudy, it was taking longer to land while Alexei was so unwell."
The lawyer heard retching noises as they urged him to drink.
Was his stomach pumped?
Omsk airport's chief doctor, Vasily Sidorus, has refused to confirm or deny this. All he would say was "There was everything."
Had they suspected food poisoning, the crew may have tried to, says Israeli intensive care expert Mikhail Fremderman. "But that wouldn't have helped in a case of poisoning with organophosphorus compounds, which is what the Germans are now talking about."
And if Mr Navalny's food or drink had been poisoned, throwing up would have posed a risk to those offering him medical assistance, as well as those cleaning up the plane later.
At 09:01 Omsk time, the plane landed.
Medical staff at the airport boarded the plane just two minutes after landing.
As soon as they had examined Mr Navalny, the medics said "this is not a case for us - he needs intensive care", Mr Nezhenets recalls.
He then heard one of the medical staff phoning for an ICU ambulance. They asked for it to drive straight on to the landing area, saying that the patient was in a serious condition.
He then heard a medic explaining over the phone what colour the plane was and telling the driver to park close to the steps.
"We waited for another 10 minutes for the ambulance to arrive," he says. "During this time, the doctors took Navalny's blood pressure and gave him an intravenous drip - but I think it was clear to them that it was of no use."
Dr Sidorus says he did not treat Alexei Navalny personally, but that his colleagues did their best to save his life.
"It was hard to understand what was going on, as he could not speak," he says. "They did everything they had to do, saved a man's life and made sure he was transferred to an appropriate hospital."
Passengers we spoke to believe the medics spent about 15 to 20 minutes examining Mr Navalny on board the plane.
He was then taken off the plane and his stretcher loaded into an ambulance, which drove straight to Omsk Emergency Hospital No 1.
The plane was re-fuelled and, after another half an hour, continued its journey to Moscow, Mr Nezhenets told the BBC.
"When we landed at Moscow Domodedovo airport, several policemen and plain-clothed men entered the plane.
"They asked passengers seated in the rows closest to where Alexei had been sitting to stay, while the rest were free to go. Alexei had been sitting somewhere in the centre of the plane, row 10 or 11."
It seemed strange to have police come on board. "At that point, the case did not look criminal. And yet, here was the security service."
'Poisoned with Novichok'
For two days, the hospital in Omsk kept Mr Navalny in its acute poisoning department. Initially they would not allow him to be flown to Germany, citing his unstable condition.
However, on 22 August, he was airlifted to the Charité clinic in Berlin and two days later German doctors said their tests showed he had been poisoned.
Doctors in Omsk, including the chief doctor of the Emergency Hospital No 1 and the chief toxicologist, insisted that no poisonous substances had been detected in Mr Navalny's body when he was under their care. They said a metabolic disorder was one potential, alternative diagnosis.
BBC Russian has asked Omsk health authorities for a comment and a detailed account of Navalny's hospital stay, but has not received a reply.
Reporting by Anna Pushkarskaya, Elena Berdnikova, Timur Sazonov, Andrei Soshnikov and Ksenia Churmanova.