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There's a plane over London with a bomb on it. You have a suspect in custody, but he is refusing to speak. Can you torture him if you genuinely believe it will save hundreds of lives?

This hypothetical scenario is just one of the moral dilemmas posed by renowned Harvard professor Michael Sandel in this documentary. Filmed in Boston, Berlin, London and Athens, it is an extraordinary philosophical travelogue which combines the biographies of three leading thinkers from the Enlightenment and the Ancient World with a thought-provoking examination of modern ideas of social justice and citizenship.

In Germany, Sandel goes in search of Immanuel Kant, the Enlightenment thinker who has been called the "father of human rights". According to Kant's Absolutist philosophy, torture is never permissible and it could never be legitimate to kill one person, even to save a hundred. In January 2005, in the wake of 9/11, the German government introduced a law that would permit its air force to shoot down any hijacked planes that posed a threat to the public. But Sandel asks whether it can ever be morally right for the state to kill a hundred passengers, even if that means saving thousands of equally "innocent" bystanders?

For Jeremy Bentham, the British philosopher, morality was ultimately about numbers, and in certain circumstances it could be acceptable to compromise on individual rights for the sake of the masses. After a memorable encounter with Bentham's pickled corpse, Sandel discusses the implications of his Utilitarian philosophy for the modern world. If the vast majority of Europeans disapprove of the burka, he asks, does this mean that it should be banned?

For Aristotle, the Ancient Greek philosopher, justice was neither about numbers nor about absolute rules – it was about giving people what they deserve. In Athens, Sandel visits the Agora and the Parthenon, and compares Aristotle's intensely participatory vision of democratic life with the reality of modern Greece, a society at the front line of the global economic crisis. Is participating in a demonstration an act of good citizenship, he asks? And do Aristotle's ideas about social justice and political engagement have anything to say to David Cameron's vision of "The Big Society"?

This thought-provoking film includes interviews with contemporary philosophers, politicians and thinkers from all around the globe.