RTÉ's 'Documentary on One' tells of how a forgotten Irish kidnapping irrecoverably changed the lives of three generations of one family.

In January 2012, RTÉ's Joe Duffy received a letter urging him to dig deeper into the kidnapping of Thomas Niedermeyer in December 1973.

The kidnap and murder of the 45-year-old German boss of the massive Grundig electronics plant in Belfast had been the topic of a Liveline programme.

The teenage daughters of Mr Niedermeyer answered a knock on the door two nights after Christmas 1973. Two strangers asked Gabriella and Renate to fetch their father, telling the girls they had accidentally damaged his car.

Mr Niedemeyer was then attacked and bundled into a car by the Provisional IRA in front of his daughters.

They never saw their father again and would have to wait seven years to bury him.

The abduction was the beginning of a botched kidnapping by the Provisional IRA and a tragedy for the Niedermeyers that would ultimately result in the three remaining members of the family who were left in the house that night taking their own life.

In the days and weeks following the kidnapping, Mr Niedermeyer's wife Ingeborg and her daughters waited for news.

Mr Niedermeyer's body was eventually discovered seven years later in 1980, face down at the bottom of an illegal rubbish dump.

Ms Niedermeyer moved back to Germany after her husband's body was found, but she returned to Ireland in 1990.

She booked into a Wicklow hotel and asked for a sea view and two single beds, as she found the empty double bed too upsetting.

Her body was found on the shoreline the following morning. She had walked into the sea at Greystones and drowned. Her family said she never recovered from the murder of her husband.

In January 2012, Mr Duffy mentioned this kidnapping and the subsequent death by suicide in Greystones of Mr Niedermeyer's widow.

Subsequently, he received a letter inviting him to find out what happened to the remaining two members of the family.

Tragically, it transpired that within a few years of Ms Niedermeyer's suicide, her two daughters also took their own life.

It was then that Mr Duffy approached the Documentary on One team and efforts were made to find the remaining members of the family.

The letter had mentioned grandchildren and last year the Documentary on One team found Mr Niedermeyer's two surviving grandchildren.

Almost 40 years on from Mr Niedermeyer’s kidnapping, and for the first time ever, the only remaining members of the Niedermeyer family, Gabrielle's two daughters Tanya and Rachel, speak publicly.

In Documentary on One - A Knock on the Door, these two women tell their family's story; the story of a kidnapping, and the story of what happened to their grandfather, grandmother, mother, father and aunt in the years following that night in 1973.

Documentary on One - A Knock on the Door was brodcast on RTÉ Radio 1 on Saturday 16 February at 6pm.