On their way to meet Dr. Simon True, U.N.C.L.E. agents Napoleon Solo and Illya Kuryakin are ambushed by THRUSH operatives. Dr. True has developed a process to extract gold from seawater, but as he demonstrates the discovery, he is poisoned by Randolph, a THRUSH agent. Before he dies, True utters a chilling clue: his daughter is the key to locating the hidden formula.
Dr. True, Jim Boles in the role, leaves behind four step-daughters and one biological daughter, Sandy True, Kim Darby. He explains that each of the women has a piece of what will unlock the full formula, and the four pieces must be gathered together by Sandy to reveal the path to the prize. True’s widow, Amanda True, Joan Crawford, has been involved with Randolph, but he only used her to get closer to the notes. With no further use for her, Randolph orders Amanda murdered by THRUSH, and when Solo and Kuryakin discover her body, they recruit Sandy to help track down the sisters.
In Rome, Solo and Kuryakin track down Margo, Diane McBain, who has married Count Valeriano De Fanzini, a man who keeps her naked and captive in his family estate. He married her for money he believed she didn’t possess, and Solo and Kuryakin must free Margo. With Sandy assisting, they confront Randolph and other THRUSH agents who have followed them to Rome. After a tense confrontation, Margo and the Count reconcile, and a photo of True with a fragment of the formula emerges from Margo’s possession. The team realizes this is only one-quarter of the full code, and the search continues.
Next, in London, Imogen Smythe, Jill Ireland, is arrested for indecent exposure (in a bikini), and Solo posts bail to access Imogen’s nightclub. In Imogen’s dressing room, Sandy finds a photo of her father with a different formula in the background. The pursuit continues as the team learns more about the four pieces and where they might be kept. Imogen’s nightclub becomes a staging ground for more skirmishes with Randolph and THRUSH, and an opportunistic constable who admires Imogen helps them escape.
The quest then moves to the Swiss Alps, where Yvonne, Danielle De Metz, resides at a hotel and is in danger of eviction. Randolph—the one step ahead—pays Yvonne’s debts and presses her to hand over the photo of her stepfather. Solo and Kuryakin clash with THRUSH operatives on skis, while Sandy uncovers another photo. Yvonne bonds with her wealthy boyfriend, Carl von Kessen, Curd Jürgens, and reconciles their relationship as the search widens. Randolph intercepts the team on an U.N.C.L.E. jet, seizes three photos, and parachutes away, leaving the others to die. Sandy frees Solo and Kuryakin, and they land the plane safely.
Meanwhile, Dr. True’s fourth photo has been published in a magazine. Yet, scientifically, the assembled letters still make no sense. When the four formula segments are rearranged, they spell out a clue: “Japanese Lullaby.” Sandy recalls a Japanese man who sang her lullabies in childhood, and THRUSH soon kidnaps her and transports her to Japan.
In Japan, Sandy escapes with help from geisha and, with Solo and Kuryakin, locates the Japanese man who safeguarded the formula. He provides the missing piece, but Randolph and other THRUSH agents, who have trailed them again, overpower the trio and seize the full formula. Randolph transports the captives to a THRUSH facility at a polar base, where the plant has been repurposed to manufacture gold from seawater. The goal is to keep the team alive long enough for them to witness THRUSH’s triumph, but Solo and Kuryakin sabotage the operation using explosives hidden in Solo’s shoelaces. They escape their cell, destroy the gold-making machinery, and Randolph is killed, coated in gold dust.
With Randolph dead, Solo, Kuryakin, Sandy, geisha Reikko, and U.N.C.L.E. head Alexander Waverly travel to London for the double wedding of Imogen and the constable, and Yvonne and Carl. The Count and Contessa are also present, and it’s revealed that, in the absence of a father, U.N.C.L.E. has funded the weddings for the sisters, tying up the mission with a final note of reconciliation and cooperation.
