Adrian Messenger [John Merivale] believes a series of seemingly unrelated “accidental” deaths are linked murders. He asks his friend Anthony Gethryn [George C. Scott], recently retired from MI5, to investigate, and provides a list of the victims’ names. Soon after, Messenger’s plane is bombed and crashes into the sea while he is en route to collect evidence to confirm his suspicions. With his dying breath, he tells a fellow passenger, Raoul Le Borg [Jacques Roux], the key to the mystery, though his exact meaning is garbled.
Raoul Le Borg survives and turns out to be Gethryn’s old World War II counterpart in the French Resistance. They join forces to investigate Messenger’s list and decode his cryptic final words. Lady Jocelyn Bruttenholm [Dana Wynter], Messenger’s widowed cousin and a former love interest of Gethryn, soon joins them. Le Borg becomes strongly attracted to Jocelyn, sparking a tense dynamic as the trio begins to piece together Messenger’s clues.
From Messenger’s unpublished manuscript, the investigators glimpse important war-time information, but the killer has already been there, removing and re-typing several pages and stealing key data. Jocelyn’s account of the man in the apartment differs from Le Borg’s sighting at the airport, leading Gethryn to conclude that the murderer uses realistic masks to disguise his face.
They next seek out the only living person on the list, James Slattery [Robert Mitchum], but they are told by his invalid twin brother, Joe Slattery, that James died years earlier of a heart attack. That night, however, Joe sees the masked killer, who pursues him, knocks him out, and pushes him into the river to drown. The following day, Joe’s mother reveals that “Joe” was really James impersonating his deceased brother to collect a disability pension, a shocking confirmation that the list hides a deeper conspiracy.
With new whispers of a shared war-time past, Gethryn and Le Borg learn that all the victims were prisoners in a Burma POW camp where a Canadian sergeant betrayed his comrades to escape. This revelation points to the Canadian killer in their midst, a man poised to rise to prominence by erasing witnesses. The identification tightens when Messenger’s clue points toward Jocelyn’s Bruttenholm lineage, a landed family famed for fox hunting, and the killer’s motive becomes clearer: he must not be recognized as the one who betrayed fellow prisoners.
George Brougham [Kirk Douglas], newly unveiled at the Bruttenholm estate fox hunt, emerges as the culprit. He has already killed to protect his secret and is ready to claim the inheritance that will solidify his position. In a bid to derail the investigation, Brougham sabotages the next morning’s hunt by laying a drag over the fields and placing a hay tedder in a hidden blast zone where Gethryn would be endangered. The crowd gathers as the hunt resumes, and Gethryn, who had anticipated danger, reveals that he had moved the tedder earlier that day and plans to use the lead fox hound to track the killer.
Disguised once more, Brougham attempts to flee on Derek Bruttenholm’s horse, a gift from Messenger and trained by Romanian gypsies. Derek, who cues the horse with a Romanian command, halts the escape and exposes Brougham’s ploy. The horse’s sudden stop—followed by the fatal misstep into the trap—leads to Brougham’s downfall, as he is impaled on the very device he intended for Gethryn. The danger is neutralized, and the truth about Messenger’s list, the masks, and the wartime betrayal is finally laid bare, restoring some measure of justice as the survivors move forward.
