He had a coach accident in 1856, after which he lost his memory. As he was a police officer, he had to continue with his investigations, without telling about his lost memory. After the accident he met Hester Latterly, a Crimean War nurse, in unfortunate circumstances. From that day on, Hester and Monk became closer and closer, as she was the only one who knew about Monk's memory issues.
In the second book, A Dangerous Mourning, Monk was fired from the police for insubordination and had to become a private investigator. Lady Callandra Daviott (Hester's best friend) financed his private investigations.
Sir Oliver Rathbone was his love rival (he too wanted to marry Hester) and judicial advicer in his cases; finally Hester and Monk got married, and kept solving cases together.
William Monk is a very clever man who wields irony and sarcasm with considerable skill while remaining obstinate, proud, and impulsive. This normally disastrous combination of attributes (which caused his firing from the Police force) is offset by his intelligence, unswerving sense of justice, and humanity to those he deems worthy. As to be expected, he is inwardly highly emotional and this passion drives his excellence with the single-minded determination of the obsessed. He will solve the case, he will see the evildoers brought to justice, and he will be fearless in doing so, come what may.
While Monk lives in the Victorian era, his disregard for social conventions (openly suspecting the gentry instead of the servants in A Dangerous Mourning, and consistently ignoring class distinctions) imbues him with the power of a True Believer and gives him access to multiple layers of society, which aids his tireless efforts.