Many critics complained that the film's ending - which appears to show Tim to be insane, and therefore (perhaps) the whole story thus far to be a fantasy (possibly drug-induced) - was suddenly imposed and unsatisfactory, and some sources suggested that it might have been the result of last-minute re-editing. However, there are hints quite early on that the narrative is not as straightforward as it seems to be - the dead body of Tim's aunt is discovered by Juliet, who appears to be a complete stranger to Tim, and yet, when he gets back to England, she has suddenly become his fiancee, although there have been no scenes between them of a romantic nature at all, and his time does seem to have been fully occupied with his investigations. This mysterious plot-lacuna is never even referred to, much less explained.
During production on British Rail's Waterloo & City tube line, a train ran into the buffer stops at the end of the line at Bank station. David Hemmings, Gayle Hunnicutt, and ten extras suffered minor injuries.
The cast-list at the end of the film lists the octogenarian actress Hilda Barry as "Miss Dacey", a character who is referred to in the dialogue, but never actually appears in the film itself.
The author of the original novel, John Bingham, was not only a well-known writer, but Lord Clanmorris.