It was under these circumstances that Sophia Dorothea re-made the acquaintance of Swedish count Philip Christoph von Königsmarck, with whom her name is inseparably associated. The two first met in Celle when he was sixteen. They flirted innocently, and traced their names on the palace windows with the words "Forget me not". On 1 March 1688 he reminded her of their previous acquaintance, and the two renewed it. George Louis' younger brothers loved the count and brought him to Sophia Dorothea's salon in the evening to cheer her up. For the two years he stayed in Hanover, there was no reason to believe their relationship was anything but platonic. He left for a military expedition to the Peloponnese in 1690 — it was a disaster. On his return, the relationship between him and Sophia Dorothea intensified. They began sending each other love letters which suggest that their relationship was consummated.
In 1692, the early letters were shown to the newly appointed Elector Ernest Augustus (Sophia Dorothea's father-in-law). Since a scandal might have threatened his new status, the elector sent Königsmarck to fight with the Hanoverian army against Louis XIV. Other soldiers were given leave to visit Hanover, but he was not. One night Königsmarck deserted his post and rode for six days to visit Hanover. The day after arriving, he called on Field Marshal Heinrich and, confessing his breach of duty, begged for leave to stay in Hanover. It was agreed, though Heinrich suggested that the affair be ended or that Königsmarck leave the country. Ernest August exiled Königsmarck.
George Louis criticised his wife over her affair, and she criticised him for his. The argument escalated to the point that the prince threw himself on Sophia Dorothea and started tearing out her hair and strangling her, leaving purple bruise marks. He was pulled off her by her attendants.
Königsmarck presumably was killed while assisting her in a futile attempt to escape from Hanover. In 1694 the Count disappeared (several guards and the Countess Platen confessed on their deathbeds to being involved in his death); the princess was divorced by her husband and nevertheless imprisoned at the Castle of Ahlden. She remained in captivity until her death more than 30 years later on 13 November 1726. Sophia Dorothea is sometimes referred to as the "princess of Ahlden." Her two children were the British king, George II, and Sophia Dorothea, wife of Frederick William I of Prussia, and mother of Frederick the Great.
Sophia's infidelity to her husband is not absolutely proven, as it is possible that the letters which purport to have passed between Königsmarck and herself are forgeries. George II was very disturbed by the imprisonment of his mother, and it was one of a number of reasons that contributed to the relationship of mutual hatred between him and his father.