Unneutral Murder

It is 1943, and wealthy criminologist Amos Lee Mappin is on a government mission to neutral Portugal, to retrieve some anti-nazi refugees who are destined for USA.
Naturally, he runs into opposition from German agents; what’s unusual for this type of crime/thriller is how he accumulates proper evidence against the opposition, to their eventual undoing. There is of course a hairs-breath escape, and other action.