Clues

  1. The Baroness was pretending to be sick. 
  2. The Baroness was pregnant. 
  3. The Baroness had cheated on the Baron. 
  4. The Baroness's housemaid gave the child to an orphanage. 
  5. The Baron and his wife were threatened by the Baroness's child. 
  6. The child found out about his origins. 
  7. The child wanted revenge. 
  8. The Baron took the side of his wife's child. 

Solution

The Baron used to be a military general. For some time, he was stationed in India, where he lived with his wife. He didn't know that she had started an affair with a handsome Indian. After they returned to England, the Baroness realized that she was pregnant. She knew it would be clear that the Baron was not the father. She decided to hide the pregnancy. 

In collaboration with a medical doctor and her trusted housemaid, she simulated a tropical illness, which supposedly necessitated that she stay under lots of covers in a separate bedroom. After a few months, she gave birth to a child and ordered her maid to give him away to an orphanage. 

For twenty years, the boy lived a poor life, not knowing his true origins. He decided to uncover his origins. He found out that it was a Baroness's housemaid that gave him to the orphanage. He also found out that the Baroness hadn't left her house for a few months after returning from India. 

He figured out the truth and, seeking vengeance, sneaked into the Baron's house. Everything would have ended horribly, but the Baron, having heard the whole story from his wife, took the boy's side. He promised him financial help and declared he will punish the woman who was responsible for his miserable life.