This is quite a good film describing the life and work of Helena Kowalska, who entered a convent as Sister Maria Faustyna and experienced divine visions of Jesus Christ expressing emphasis on His ability to accord Divine Mercy to believers, even in the most extreme circumstances, and asked her to commission a portrait of Him in which such merciful attributes literally emanate from him in the form of rays of forgiveness. Sister Faustyna was ultimately canonized and is now a Saint of the Catholic Church.
The story is initially presented in "docudrama" style, with actors portraying the principal participants (Christ Himself, Saint Faustyna, Father Michal Sopocko, her confessor who encouraged her to write a diary of her experienced and ultimately helped commission the portrait, and other participants. The second part of the film is devoted to interviews with various spokespersons explaining in further detail the concept of Divine Mercy.
The film itself is quite good: the production values are excellent and the actors do a good job (with special mention to the actress portraying Saint Faustyna).
However, the film clearly speaks the strongest to those viewers already convinced of the truth of Saint Faustyna's visions and the concept of Divine Mercy. A significant portion of the film is given over to pseudo "scientific" proof of the visions, including a somewhat ludicrous sequence in which it is "proven" that the head of Christ as portrayed in the painting corresponds to the image of the Shroud of Turin, thereby demonstrating without doubt the veracity of her visions. There are many possible explanations for this (after all, the popular image we all have of Christ is fairly universal).
It seems to me that the film would have been better off either simply expounding further on the concept of Divine Mercy itself or taking a different approach to convincing those unaccepting of its virtues, e.g. by showing how appealing a concept of univeral forgiveness is, rather than trying -- and in my view failing -- to provide a "scientific" basis for it.
In short, if you are Catholic and especially if you find the concept of Divine Mercy to be particularly comforting, you will probably find the film to be an uplifting and emotional experience. If you are not, then I do not think this will be the film that convinces you.