On Her Majesty's Secret Service continues the saga of Ernst Stavro Blofeld which began in Thunderball. Secret Service agent James Bond, operative 007, is in the midst of "Operation Bedlam," hunting for Blofeld after his criminal agency gained control of two nuclear devices the year before and threatened the existence of the western world. Followng a year of frustrated leads, Bond doubts SPECTRE still exists, but MI6 won't allow him to abandon the search. Finally, Bond begins to write his letter of resignation. His voice is no longer being acknowledged by his superiors.
While writing the letter, he encounters an attractive young woman on the road. Later that day, while gambling, he sees the same woman and saves her from defamation or worse by paying a debt she couldn't afford. This woman is Contessa Teresa di Vicenzo. She introduces herself to him as Tracy. Attracted and intrigued, Bond follows her around the next day. He ends up preventing her from attempting suicide, but they are both taken by a couple of professional thugs and taken to an office building. Bond is introduced to Marc-Ange Draco, Tracy's father and leader of the infamous European crime syndicate Unione Corse. Much to his surprise, Draco offers Bond one million pounds to marry his daughter, believing love the only way to prevent her from further attempts at suicide. While Bond won't accept the offer, he does stay and continue to seduce Tracy. Her mental state gradually recovers.
During his stay with Tracy, Bond becomes better acquainted with Draco. Draco pulls some strings to help Bond on his mission, telling him that Blofeld is hiding out in Switzerland. With this information Bond returns to England to report in with his superiors. All thoughts of resignation are now abandoned. His commander, M, gives Bond an additional lead. The royal establishment of the College of Arms, has received a request from Blofeld for the title Comte Balthazar de Bleuvile. Following this lead, Bond journeys to the College of Arms. While there, he learns that a possible ancestor of his, Sir Thomas Bond, possessed the family motto "The World is Not Enough." The College of Arms instructs Bond that they need a confirmation of Blofeld's legitimacy to the claim of count. Consequently, Bond is presented the alibi he needs to travel to Switzerland in search of Blofeld.
Blofeld has established a "research facility" on top of Piz Gloria, in the Alps. Bond travels there under the alias Sir Hilary Bray, sent by the College of Arms to examine Blofeld's earlobes. The family lineage Blofeld claimed to belong to have the peculiar genetic trait of a lack of earlobes. Knowing this, Blofeld had previously commissioned plastic surgery to remove his earlobes. Now he both fits the ancestral claim and proves harder to indentify by the various authorities that are hunting him.
At the facility, Bond meets twelve beautiful young European women. They have all come to Blofeld to be cured of their various allergies. Supposedly they have been. In reality, however, Blofeld and his second in command, Irma Bunt, have been brainwashing the women while they sleep. They have been unconsciously programmed to transport biological weapons back to Birtain and Ireland and to deploy them upon the local agriculture. Blofeld's evil scheme is to create a food scarcity and cripple the European economy. Bond discovers the scheme.
Bond's alibi is found out, so he takes off on a frantic ski course down the mountain. Chased by henchman, Bond quickly loses his strength. He can't fight all of them. Suddenly, he stumbles upon Tracy near the base of the mountain. She had come to Piz Gloria after interogating her father about Bond's departure. The two manage to escape to the Swiss airport together. Defeated, Bond returns to England to scheme up way to capture Blofeld.
Tracy plays no small role in the success of his mission. After their mountain escape, Bond is falling madly in love with her. She convinces her father to use the Union Corse to attack Blofeld's facility. They launch an air assault, destroying the entire stronghold. Blofeld manages to escape on a bobsled, but Bond chases him. Unfortunately he can't capture the master criminal. In Germany Bond and Tracy are married. The happy couple drive to their honeymoon. In a tragic twist of fate, Blofeld and Bunt drive by with a machine gun. Bond is miraculously spared, but the bride is not as fortunate. In the end, Bond is alone once more. Now he is determined to exact revenge upon Blofeld.