Russian spy plot thickens

This Cold War spy thriller is really heating up.

The investigation into the attempted murder of an exRussian spy and his daughter in England over the weekend expanded Thursday to include the deaths of his wife and son — as police revealed that 21 other people have been treated in connection with the nerve-agent attack.

Police on Thursday cordoned off the graves of former double agent Sergei Skripal’s wife, Lyudmila — whose death certificate says she died of cancer in 2012, and son Alexandr, who reportedly died of liver failure while on vacation in Russia last year, the Salisbury Journal reported.

Their deaths are now part of Scotland Yard’s official investigation into the attack on Skripal (inset) and his daughter Yulia, who simultaneously succumbed to the poison on a park bench in Salisbury on Sunday, the newspaper reported.

Meanwhile, local police revealed the full fallout from the poisoning, saying 21 other people have been treated in the hospital as a result.

They include an off-duty doctor and nurse who stopped to help the father and daughter as they struggled to breathe on the bench, and the first police officer to arrive on the scene, according to the Journal.

Police on Wednesday announced that Skripal and Yulia’s previously mysterious illness was the result of a murder attempt with a nerve agent — further fueling speculation that the Kremlin may have been behind the attack.

Skripal is a former Russian military intelligence officer who was jailed for selling secrets to Britain, before being freed and sent to England in a 2010 prisoner swap that included New York-based Russian spy Anna Chapman.

Moscow has denied it has any connection to the attack.