How does the witcher 1 lead into the witcher 2?

Ive never played 1 but just started 2. After the tutorial he is captured by guards and is being beaten whenever he wakes up. Eventually some guy comes in and tell them to move geralt to the interrogation room. Now I have no idea who this is and he seems important. So i watched some recaps and it ends with geralt killing an assassin and saving the king. The assassin is a witcher. But thats where it ends. So why is he captured and who is he captured by? Sorry im just confused. EDIT: Also bonus question in the first game he doesnt remember anything is that how this world starts or is there some game or movie before the witcher 1?