Alexander Litvinenko (David Tennant,), a former Russian spy, Kremlin critic and British citizen, lies in a London hospital bed, his condition deteriorating rapidly. Detectives are summoned to take his statement – he claims to have been poisoned and points the finger directly at President Vladimir Putin. Litvinenko dies just a few days later, surrounded by his family. When it’s confirmed that he had been poisoned with polonium-210 – the deadliest substance known to man – the gravity of the situation becomes clear. Facing an unprecedented chemical attack in the heart of London, the police race to seal off potentially contaminated sites, while detectives plunge into a high-stakes murder investigation, tracking their elusive suspects from London to Moscow.