Pétur (Hilmar Jónsson) and Ásta (Margrét Vilhjálmsdóttir) lead a very comfortable existence. They earn a good living and are expecting a long-awaited second child. As the film opens, Ásta is having an ultrasound just before they are due to leave for a tropical vacation. At the same time, their young son, Örn (Aron Brink Sigurjónsson), faints during a soccer game and is rushed to hospital. When Pétur arrives to bring him home, his entire world is overturned: he learns Örn's blood type doesn't match his or Ásta's - meaning Örn is not his son. How he deals with this news forms the crux of Árni Ólafur Ásgeirsson's chilly and often harrowing Thicker than Water. With one of the cornerstones of his world pulled out from underneath him, Pétur seems determined to demolish what's left. He checks into a hotel and embarks on a lengthy drinking binge, all the while refusing to discuss what's going on with anyone but Ásta, who stubbornly denies any wrongdoing. Meanwhile, Pétur refuses even to acknowledge Örn. Eventually deciding to move on, he starts an almost comical affair with his much younger assistant, Anna (Laufey Eliasdóttir), but their age difference doesn't help. Nor does the presence of Anna's roommate, the jealous Rabbi ( Jón Páll Eyjólfsson), a club kid who insists on calling Pétur "Pops." In the early going, as it virtually quakes with rage, the film has a decidedly unnerving feel. We see things entirely from Pétur's perspective and are baffled by Ásta's refusal to respond to his accusations. The battle of wills is transfixing and, when we see the impact on their son, horrifying. But as the film proceeds, Pétur emerges as the problematic figure. We wonder how long he will wallow in his rage - and whom he'll lash out at next. Jónsson and Vilhjálmsdóttir deliver stunning performances, capturing their characters' stony intransigence while conveying the world of hurt lying beneath it. Directed with remarkable restraint and sensitivity by Ásgeirsson, Thicker than Water recalls W.H. Auden's observation that even schoolchildren learn, "Those to whom evil is done / Do evil in return" - a truth few adults care to remember or act upon.