It should have been a routine crossing of the Irish Sea between the Scottish port of Cairnryan and Belfast in Northern Ireland.
But it turned into a desperate dash to save a woman and a baby, thought to be just days old.
The pair fell overboard the Stena Line Superfast ferry and into the cold waters of Belfast Lough.
A major search and rescue operation was launched when the alarm was raised just after 6 o'clock on Wednesday evening. It involved all the emergency services, including the RNLI, the Maritime Coastguard Agency, and the PSNI and Irish Coastguard helicopters.
The woman, thought to be the child’s mother, was recovered from the water and taken to the Royal Victoria Hospital in Belfast for treatment.
A spokesman for the Northern Ireland ambulance service said they treated the woman who had been in the water for some time. The spokesman said "she would have been hypothermic." Her condition in hospital is described as stable.
A full police investigation was launched. PSNI officers boarded the ferry and efforts are underway to establish exactly what happened.
The Stena Line terminal remained closed on Wednesday evening and all sailings to and from
Belfast have been cancelled. Passengers are advised to call the 24hour Stena Line ferry check service for updates on sailings they have booked on. The number is 08705 755 755.
Belfast Port was closed to all ferry traffic in a bid to ensure the operation continued but was later reopened.
It will recommence at first light on Thursday morning but would seem certain to now become a recovery operation with the emergency services and maritime agencies focusing on the heartbreaking task of recovering the baby's body.
While all efforts are being made to find the child, hope is fading fast.