The film, set against the backdrop of March 1943 during the height of World War II, centers on the critical operations at Bletchley Park, Buckinghamshire, where British crypt-analysts face a daunting challenge. The Nazi U-boats have altered their code reference books used for Enigma ciphers, resulting in a significant disruption of essential naval signals intelligence. With the pressing need to decode the "Shark" cipher once again, the team realizes they must track U-boat positions to avert disaster.
The story opens with Tom Jericho (Dougray Scott), who has just returned to Bletchley after a month’s absence to recover from a nervous breakdown caused by a tumultuous relationship with a colleague, Claire Romilly (played by Saffron Burrows). The head of the facility, Skynner (Robert Pugh), holds a disdainful view of Jericho, and with the looming threat of Admiral Trowbridge (Corin Redgrave) taking over the crypt-analyst program, Skynner is visibly anxious.
Upon his return, Jericho eagerly seeks out Claire, only to discover that she has mysteriously vanished in the days prior. To find her, he enlists the aid of Claire’s housemate, Hester Wallace (Kate Winslet), as they navigate a trail of clues that may unravel her fate. In this high-stakes scenario, time is of the essence—a convoy of thirty-seven ships from America is on the verge of being ambushed by U-boats, and the code-breakers have less than four days to crack the modified Shark cipher.
Jericho passionately explains to naval authorities that the Shark Enigma Machine creates an astronomical 4 billion billion combinations, rendering any plaintext almost unrecognizable. Even with the same settings, the ciphertext will differ each time, further complicating their mission. While they possess an Enigma machine, they lack a reference code for the day’s cipher settings. The last time they managed to decode a similar code, it took them ten months.
In this tense environment, Cave (Matthew Macfadyen), from Naval Intelligence, becomes an ally to Jericho, hinting that he is aware of how Jericho had previously cracked the code using a German weather code book. As Jericho searches through Claire's belongings, he uncovers stolen German transmissions hidden beneath a floorboard—material crucial for cracking the new codes.
Hester and Jericho, formally addressing each other in their quest, work together to decode the intercepted messages stolen by Claire and speculate about her motivations. The enigmatic ADUADU appears in their findings, indicating a German signals unit based in Ukraine. Their investigation reveals a bigger conspiracy that may compromise their mission as well as their lives.
As the tension mounts, the urgency of their task escalates when Cave announces that the German U-boats have gone radio silent, hinting at an imminent attack. Jericho and Hester deduce that previous intercepted messages may still be available at a local receiving station, leading to a daring plan to retrieve critical information for their code-breaking efforts.
Through a series of suspenseful encounters and revelations, they evade capture by the security services while piecing together the puzzle of Claire's disappearance and the code's significance. Hester's analytical skills shine as she identifies coded messages linked to a grave situation involving Polish names—an operation too sensitive to share for fear of political ramifications with their Soviet allies.
As U-boats initiate their assault on the convoy, the clock ticks down, resulting in significant loss of life and sparking a race against time to decipher the messages. The gravity of these communications weighs heavily on Jericho and Hester as they uncover the tragic intertwined fates of Claire and Jozef 'Puck' Pukowski (Nikolaj Coster-Waldau), who plays a pivotal role in the unfolding drama.
In an emotionally charged climax, Jericho confronts the harrowing truth about Claire’s involvement and her ultimate fate. In an extraordinary twist, they trace Jozef, revealing his treacherous actions and finding themselves in conflict with their own government agents. After a poignant journey, Jericho learns that Claire is alive, rejecting the notion of betrayal.
Ultimately, the film concludes with a glimpse into Jericho’s life post-war, as he is now married to Hester and looking forward to starting a family, reflecting on the life-altering events that unfolded amidst the chaos of war. In the end, fate brings an emotional reunion with Claire in 1946, leaving viewers reflecting on resilience, love, and sacrifice amidst one of history’s darkest times.
