Not a light film! It is about Churchill becoming prime minster and Dunkirk. I have not watched Dunkirk so, I do not know how it correlates. It is the kind of film that makes you intrigued about the historical events it covers.
Visually it is dark; a lot is set in the war rooms which were underground. Having visited the war rooms it captures their atmosphere well (incidentally they are worth a visit).
Enjoyed scenes with Churchill and the King, his wife and his secretary.
Highlight was the underground train scene, which I will not spoil here but the second review linked to below, goes into more detail. Also the following link is where the director discusses the scene and that definitely contains spoilers.
Perhapas not a film for everyone or at all times but interesting if interested in the history. I pretty much agree with these reviews. Some may find it tedious but there are good points.