As the aeroplane touched down at Nice airport, I wondered how much I would remember of the most incredible holiday of my life, when I was just 16. I had described the yacht Christina to my children, Alexander, 15, and Sophie, 13, but I knew they were unprepared for her splendour.

Monte Carlo is probably the best known harbour in the world for luxury yachts. But I couldn’t possibly mistake the newly restored Christina (now called Christina O), tied up along the quay and attracting as much attention as any superstar. It is 42 years since I last saw her. My grandfather Winston Churchill had invited my mother and me to accompany him on a summer cruise as guests of the Greek shipping tycoon Aristotle Onassis and his wife Tina.

None of us, of course, could have any idea just how extraordinary this cruise would prove: a fateful and glamorous voyage that saw the unravelling of two marriages and the start of one of the legendary affairs of the 20th century, between our host Aristotle and our fellow guest Maria Callas, the great Italian opera singer, who had been accompanied on board by her ageing husband and manager Battista Meneghini. Agatha Christie could not have invented a more glittering cast nor idyllic setting for the drama about to unfold.

To read Celia Sandys' full account of the start of the legendary love affair of Aristotle Onassis and Maria Callas, see this week's HELLO! magazine, on sale now.