The critically ill former Russian double agent Sergei Skripal is pictured on most of the front pages - along with police in yellow protective suits examining the area where he was found in Salisbury.
The Daily Mail says he may have been the target of a "revenge hit" by former Russian colleagues. He is said to have recently told police he feared for his life.
Sources tell the Times nothing is being ruled out - including the illicit administering of "something very sinister".
The Daily Telegraph says an unknown substance was removed from the scene, wrapped in protective layers.
Wiltshire Police say they are investigating whether a crime has been committed.
But Garry Kasparov - the former world chess champion and a prominent critic of President Putin - tells the paper: "After the UK's pathetic response to the assassination of Alexander Litvinenko - why wouldn't Putin do it again?"
"Another scandal with a Russophobic tinge is brewing" is how the story is introduced by the Moscow daily Komsomolskaya Pravda. It suggests UK intelligence services may be involved in the incident in order apparently to discredit Russia and prevent the normalisation of relations with Britain.
The website of the Russian broadcaster RT says the British media are "rushing" to compare the incident with the death of the former Russian security officer Alexander Litvinenko from radiation poisoning in 2006.
Sir Bradley Wiggins is the focus of a story in the Times which identifies him as an investor in a tax avoidance scheme that has been described as "notorious" and "shocking" by MPs.
The Cup Trust, which was closed down last year, used a charity as a front to avoid tens of millions of pounds in tax. The arrangement is not illegal and a spokesman for Sir Bradley tells the Times he has paid all his due taxes and has no open inquiries with HMRC.
The Financial Times leads with a claim that transatlantic routes operated by British airlines are at risk, after negotiations with the US for a post-Brexit flights deal got off to a tough start.
The current arrangement stems from the UK's membership of the European Union. The sticking point is said to be US rules about foreign ownership as British Airways and Virgin Atlantic have large overseas shareholdings.
The Sun is deeply unhappy about a call for the food industry to cut the calorie content of popular dishes.
It detects a snobbery about what it calls the "professional meddlers" at Public Health England, who it accuses of seeking to victimise and bully the poorest for what they eat.
The Daily Telegraph heralds the arrival of the reforming Saudi Crown Prince Mohammed Bin Salman, who is due to visit the UK this week.
He has given an interview to the paper, in which he emphasises the importance of defence links.
The Telegraph asserts that Saudi Arabia is important to the UK in many other respects too - as a market, a potential investor and a bulwark against Islamic extremism. As such, the paper says, he is a welcome guest on British shores.
The former BBC television presenter Bill Turnbull is shown on the front of the Daily Mirror and the Daily Express with the revelation that he has been diagnosed with prostate cancer.
In an interview, he says he knows his time with his family has been shortened, and speaks of his regret he did not get checked sooner.
The Mirror urges its readers to "listen to Bill" and go to the doctor when the body starts to present warning signs.
Mind the glass
The problem of birds colliding with the glass windows of office buildings is a fairly common one. It is less heard-of with human employees.
But such is life at Apple's new spaceship-style headquarters in Cupertino, California where - according to the Guardian - workers distracted by their phones keep walking into glass windows and injuring themselves.
A building inspector is said to have warned that the Norman Foster-designed circular building would leave employees struggling to tell the difference between windows and doors.
The Guardian says in the first month after opening, three people walked or ran into transparent partitions so hard they needed emergency treatment.