“If we were in another state,” mutters one of Cyril Tuschi’s interviewees, “I could be a lot more open with you”. And therein lies the problem. A vital but ultimately frustrating documentary about one of Russia’s most controversial oligarchs, Khodorkovsky does its best to shed light on the mind-meltingly shady dealings between Vladimir Putin and the man who went from being one of the richest businessmen in the world to a Siberian prisoner. The constant political road-blocks can’t help but take their toll on the feature’s punch, but it’s fascinating stuff all the same.