How do we cope when people we love commit terrifying acts of violence, which leave innocent people dead and injured? How do we come to terms with what they have done? And how do we remember them?

Despite a central premise, pregnant with possibility, what emerges here is an agonising and protracted labour.

The film starts well, as radio broadcasts announce that Japan will hold a day of remembrance for those who died four years ago, as the result of an act of terrorism, committed by a cult group, The Arc of Truth. It has all the hallmarks of great cinema - a fact based story, four ordinary people brought together by a single catastrophic event and the additional twist that the central characters are related to the people responsible. On top of that, there is the confounding enigma of cults and why they do what they do.

Each year, one woman and three men meet to remember their loved ones. In an obvious plot contrivance their vehicle is mysteriously stolen, leaving them to face a long night of reflection and reminiscence, which they are forced to share with the only surviving cult member, who himself is struggling with his own demons and a healthy dose of survival guilt.

It's a great story, based on an actual event - The Tokyo Gas Bombing - which left deep scars on the Japanese psyche. But the lack of background information, the drip drip revelations and endless pregnant pauses combine to produce one long silence.

Writer/director Hirokazu Kore-eda employs effective camera work in the flashback sequences, using deep longshots to intensify the unchanging nature of memory, while the contemporary action is filmed hand held, emphasising the jittery, unstable reality of the present. These scenes are the most interesting, because, like the characters, we want to know the answer to the question: why?

In flashback, we watch a wife angrily pushing her husband out the door, as he prepares to abandon her and their child. We see a husband, angry and baffled, as his wife sits serenely, planning to leave him. There is the brother, who drops his medical studies, and the sister, who deserts her family, all for some greater goal that we grapple to understand.

This film is a demanding watch, with a run time of just over two hours. The pace is painfully slow. In the first half hour, we meet the characters. It takes 30 minutes to get to the memorial site. And so it goes. Only after 90 minutes, as the characters reminisce, do things start to get interesting. Alas, for me, this is too little, too late.

Some judicious editing would have improved the film, without losing the point, and might have kept the audience from wishing for a fast forward button. Don't expect distractions, light relief or high drama. Instead, fasten your seatbelts for the long haul.