This film is early in the Tracy-Hepburn canon, and not widely regarded as one of their best efforts. The thing is, coming out so shortly after their landmark Woman Of the year, it isn't properly a Tracy-Hepburn vehicle so much as a George Cukor-Donald Ogden Stewart movie in which they happen to appear.
It is the story of a newspaperman (Tracy) out to investigate the circumstances behind the death of a much beloved American hero, meets and falls in love with the man's widow (Hepburn)who, along with everyone else whoever knew the man, seems to be harboring some dark secrets as to the true nature of his character. The film owes some obvious debts to Citizen Kane in being the inside scoop on a recently deceased man presumed to be great but who was in actuality something else altogether. In its somber mood, forbidding mansion, enigmatic and generally paranoid aspect, Keeper of the Flame suggests Kane in many regards, but is, to be fair, its own film.
Tracy and Hepburn play their roles exceedingly well. The supporting cast is well-chosen, and Percy Kilbride does a nice turn as a cab-driver; while Margaret Wycherly is scarifying as the dead man's mad mother; and a young, Aryan-looking-as-all-getout Forrest Tucker scoots about on a motorcycle like he'd join Hitler's minions at the drop of a hat. Richard Whorf in what at the time must have seemed a 'daring' performance, plays a fussy secretary to the dead hero in a manner which suggests a combination of repressed mania and strong homosexual tendencies. His character is wholly unbelievable but awfully fun to watch.
The movie has a dark, gothic cast to it, and was obviously filmed on a studio back-lot, but the result is not so much unreality as the suggestion of a fairy tale or a fable strangely consistent with the film's intent, and hence satisfying, making its woods and country roads look at times like a weird and twisted perversion of a Norman Rockwell painting.