The indefatigable Robert Guédiguian returns to the highminded thriller style that proved successful with his 2009 picture The Army of Crime, which unveiled local complicity in the betrayal of a wartime resistance cell in German occupied Marseilles. This new film, for which the original French title is a slightly more snappy Une Histoire de Fou (A Story of Madness), jumps forwards three decades, to Marseilles in the 1970s, and takes as its subject the wave of bombings and assassinations perpetrated by Armenian radicals against Turkish interests, in response to the genocidal killings of Armenians during and after the first world war.

With his Armenian heritage, this counts as deeply personal territory for Guédiguian; though you sense that the director’s uncompromising political sternness makes it difficult for him to fully plant a flag. Nevertheless, he has produced a film that both acts as a useful primer for understanding the decades-long grievance that the Armenian genocide produced, and discusses the peculiar politics of direct action terror in the 1970s.

Don’t Tell Me the Boy Was Mad begins with a black-and-white preface, describing the assassination of Talaat Pasha, the Ottoman minister generally considered to have initiated the 1915 massacres, by Soghomon Tehlirian in Berlin in 1921; he was acquitted by a German court who, somewhat ironically, were outraged by Tehlirian’s accounts of Turkish-organised death marches and concentration camps. The film then abruptly cuts to the 1970s and the Armenian diaspora in Marseilles where we home in on a storekeeper called Hovannes (Simon Abkarian, from Army of Crime), his wife Anouch (Ariane Ascaride, Guédiguian’s wife and regular collaborator), and hotheaded son Aram (Syrus Shahidi). Fed with tales of Turkish brutality by Anouch’s aged mother, Aram joins a local group of like-minded agitators, which becomes the gateway drug of the very 70s form of urban terrorism. Soon Aram finds himself clutching a detonator, waiting to blow up the Turkish ambassador to France.

It’s here that Guédiguian’s takes a significant detour into more complex moral discussion. As Aram is about to push the button, a random cyclist pulls up behind the ambassador’s car; Aram makes the choice to set off the bomb anyway. The cyclist, called Gilles, is not killed, but severely enough injured to require months of operations and be largely confined to a wheelchair. Aram disappears to Beirut, there to join up with like-minded urban guerrillas and continue the campaign of terror; but racked with guilt, Anouch tracks Gilles down and offers him the family’s help, as a kind of penance. Gilles, angry and bitter, takes up the offer; after practically moving into Aram’s old bedroom, he starts to take on and identify with the Armenian cause. Meanwhile, over in Beirut, Aram swiftly becomes disillusioned with his commander’s callousness towards innocent bystanders – as Gilles once was – but can’t quite bring himself to quit for a more principled splinter group to stay with his lover, Anahit.

All this makes for a meaty two-hour-plus drama, with Guédiguian sketching in the moral dilemmas with clarity and firmness. The central debate is rehearsed again and again: can innocents ever be sacrificed for a cause, however urgent? Some of the dialogue is a little decks-clearing – Ascaride at one point quickly explains that “most Armenians abhor violence” – while the largely studio-bound sets make the film feel a little airless. It’s only when we get to Armenia in the final frames that the horizons open up. Guédiguian, none the less, has something interesting to say; his film is always good, if it’s not quite brilliant.