In a covert Romanian facility, Erik Heller evades security to rescue baby Hanna. 15 years on, father and daughter live deep in the Polish forest. Erik has trained Hanna to be an incredible killer and hunter. Yet, keen to grow beyond the boundaries of her isolated world, she begins to venture away from home. This attracts the attention of CIA agent Marissa Wiegler, who has hunted Hanna since birth.

Following her capture by Marissa's men, Hanna must fight to escape from the Moroccan CIA facility and join Erik in Berlin. Along the way, she meets Sophie, a British teenager on holiday with her family, who gives Hanna her first proper taste of the real world and the thrill of adolescence. Yet despite this glimpse at normality, the threat of Marissa and her operatives is never far behind.

Hanna and Erik are reunited in Berlin, where they hide out with his old army friends and she learns more about her father's past. However, Hanna continues to long for the normal life she glimpsed with Sophie and becomes increasingly frustrated at the restrictions her own father imposes on her. Sensing Marissa is closing in on them, Erik and his friends begin to prepare for an attack.

Erik is holding Marissa prisoner, trying to negotiate a deal to get him and Hanna safely out of Berlin. Meanwhile Hanna hides out with Dieter and his family. Desperate to know more about what her father is up to, Hanna discovers a huge secret in her own past. As an escape plan begins to form, Marissa attempts to make her own.

Sophie and Hanna's relationship begins to turn sour over their shared interest in Anton. In the fallout from this argument, Marissa arrives at Sophie's house pretending to be Hanna's mother. Hanna finds herself torn between endangering Sophie and her family, or giving up her own freedom and going with Marissa. Meanwhile, a captured Erik is brutally interrogated by Sawyer and his men.

Realising that Hanna will no longer accept anything but the full truth, Erik takes her back to Romania so she can learn more about her past. Meanwhile, Marissa begins to sense that Sawyer has not been telling her the full truth about Utrax.

When Erik reveals to Hanna the truth about Utrax, she is determined to take action. Meanwhile, Marissa attempts to find out from Sawyer what has really been going on at the facility.

As Hanna hides Clara in the vast forests of Northern Romania, the remaining trainees are transferred to an educational facility called The Meadows, where they are assigned new identities. Marissa sets about finding Clara by posing as her mother online, luring her to a hotel in Bucharest, where she is ambushed and captured by Utrax. Hanna follows and is reunited with Marissa.

Marissa and Hanna return to Paris, whilst Clara is introduced into the fold at The Meadows - but her rebellious nature causes problems, and Terri is tasked with persuading her to rejoin the programme. Pursuing Carla, Hanna finds the pharmaceutical company behind the Utrax medical implants and takes part in a drug trial, where she discovers that the drugs are bound for the trainees at The Meadows.

Hanna arrives in Barcelona, intent on saving Gelder and persuading Clara to leave Utrax. Hanna tells Clara the name and whereabouts of her mother, leaving her conflicted. Sandy has earnt Gelder’s daughter Kat’s trust and is furious when Clara backs out of killing them. She kills Gelder whilst Hanna and Clara escape with Kat. Marissa is tasked with finding Hanna by Mannion’s colleague, Grant.

Following Gelder’s murder, Carmichael arrives in Barcelona whilst Hanna, Clara and Kat hide in a hillside villa. Hanna returns to the hotel and recovers Gelder’s target list, helped out by Marissa, who follows Carmichael to the villa and blackmails him into bringing his fellow Utrax leaders to justice. Hanna allows Clara to reunite with her mother before returning to Marissa to help destroy Utrax.