The Secret Adversary serves as the beginning of the BBC series "Agatha Christie's Partners in Crime". A full-length film, I've seen it claimed that it was filmed after the ten episodes of the series, and IMDb lists the series as preceding this film, yet it's the story of how "Partners in Crime" protagonists Tommy and Tuppence meet up as adults and start their sleuthing career, and according to IMDb, at least, it aired a year or so before the series on BBC.
It's a decent but flawed film that will probably be enjoyed most by hardcore Christie fans. One main flaw is that the story is too complicated for its own good--probably the result of trying to translate a novel to a two-hour film. There are just too many characters, too many threads, and occasionally, too little explanation of characters and threads.
One major problem here is the core of the plot--maybe I'm just lacking some knowledge from the real world that I need to understand it, but the plot hinges on a political document that if found, is predicted (with little lack of certainty) to bring about a complete revolution/overthrow of England. It is never explained why this would be or how anyone could be so certain of it, and I sure couldn't deduce or intuit it from the information given. I was also confused about the implied ethics of the situation--the document seemed to be authentic, yet the "good guys" seemed to want to get it so that they could keep it a secret, so effectively, it is sanctioning the "good guys" lying about some piece of history. Maybe the document was supposed to be more like a Hitchcockian MacGuffin, but if so, it seemed like too many details were given (as well as left out).
On the other hand, it didn't help that I'm a bit hard of hearing, that I have an even more difficult time making out English spoken with accents different than mine, and that this DVD didn't have subtitles. But I could pick up most of the dialogue, and I was still occasionally confused about who someone was and why our chief characters were going where they were going and doing what they were doing. Some cuts and scene transitions were very rough dramatically, so that didn't help, either, and the pacing gets a bit draggy at times.
The other primary flaw comes with some of the performances, especially Gavan O'Herlihy, playing an American, Julius P. Hersheimmer. He is fairly awkward throughout, and he's an important character. It kills too many scenes. The stars, James Warwick and Francesca Annis, as Tommy and Tuppence, respectively, are much better--enough that I'm looking forward to seeing the rest of the series, which I'm also predicting might not be so overcomplicated and underexplained because of being written for a shorter television slot.