It was a terrible idea to bring Miss Marple into Endless Night, especially as it was done here. Apparently she went to stay with Wendy Craig's character for many, many months, including going with her on a trip to Rome, where by a massive coincidence she meets up again with the chauffeur with whom she had struck up an unlikely conversation in the street back in England. Later on, Miss Marple starts wandering in and out of the new house whenever she feels like it, including putting herself in ridiculous danger. I can't say more without spoilers.
I also can't say much about the ruining of the plot. The book is a good one and written in an unusual style for Agatha Christie. It has a surprise ending - which is mangled right out of existence here.
The acting is so so. Tom Hughes, who has the main part, mainly sleepwalks through it. In real life he is not only an actor but a model, and that aspect is very much to the fore here.
Read the book, and perhaps see the 1972 version with Hywel Bennett and Hayley Mills. It's much better than this one.