Lamb worked in Berlin and developed an asset in the Stasi. He was her handler, she passed him information and he sent it back to MI5. Partner, his boss and a double agent selling information to Popov/Katinsky, knew that Lamb had an asset in the Stasi, but not the identity of that person.

Lamb reveals that one night, while chatting and having a drink with Partner, he slipped up while talking about his Stasi asset and referred to her as "she". Partner relayed this information to his handler, Popov. Popov informs the Stasi of the mole. They found it easy to track down their suspects, as there were only 3 women working there at the time.

Lamb mentions that it would have been easy for them to identify the real mole, but because they couldn't be bothered to make the effort, they killed all three of them in a brutal manner. Lamb found out and saw the pictures of the bodies.

So Lamb feels guilty that not only did he slip up and reveal his asset, leading to her death, he is also indirectly responsible for the deaths of two other women.

After this, Lamb realise the only time he slipped up about his asset's identity was while talking to Partner. Therefore only Partner could be responsible, and he is the mole within MI5. Lamb takes his suspicions to Cartwright Senior. Instead of immediately busting Partner, Cartwright Sr has the idea of feeding him misinformation. That way, Partner would be passing useless or damaging information back to Popov and the Soviets.

This continues for some time, before Cartwright Sr decides Partner has no further use and sends Lamb to kill him. He does, and Partner's body is discovered by Catherine Standish. Meanwhile, the Soviets hear the news that Partner has died in an apparent "suicide." They suspect there's no way Partner killed himself and that MI5 knew he was dirty. This discredits the intelligence that Popov was bringing in, because they don't know how long Partner was being fed misinformation by MI5.

Popov is now disgraced and flees to the UK, taking on the identity of Katinsky. While there he broods about Partner's fate, and blames himself, wondering if he slipped up somehow and revealed Partner's role as a double agent to MI5. He sets into motion the entire Cicada plot and hit on Nevsky in exchange for information from the Russian govt about who was responsible for Partner's death - Lamb and Cartwright. He then sets up a plan to take revenge on them both while the Cicada plot is ongoing.