When DSU Garland tells Commander Hart "We're looking at the most blatant attack by a foreign power since Salisbury", she is referring to the poisoning of Sergei (and daughter Yulia) Skripal. Skripal was a former Russian military officer, and was a double agent for British intelligence agencies. They were poisoned by a Novichok nerve agent (a chemical weapon developed by the USSR/Russia). Two Russian nationals, believed to be active officers in Russian military intelligence were suspected of carrying out the poisoning. Russia has never accepted responsibility, and various diplomatic insiders followed.