Even though someone is charged with the death of the Crown Prince, the truth behind who is responsible remains a mystery to most of the people in the palace.
The Crown Prince suffered from Hyeolheogwol and constantly passed out due to fatigue. The Queen learned about it much later and then decided to keep it a secret from the King and the rest of the subjects at the palace.
The Queen Dowager and other enemies of the Queen eventually find out about the Crown Prince’s illness and choose to use it to their advantage and install a new heir.
Efforts to cure the crown prince
As soon as the Queen finds out about her son’s illness, she looks for ways that it can be treated and any other information she can find about it. Crown Prince Taein died of the very same disease, which is how the current King found himself as the next heir.
The Queen learns that all the people who treated the former crown prince have disappeared suspiciously while even the records were lost to a fire. She continues to keep the illness a secret from others and tells people that he’s suffering from a simple skin disease.
The Queen also finds out from the deposed Queen Yoon that while Taein did suffer from the same illness, he recovered and his true cause of death was murder.
When Seongnam finds out about his brother’s illness, he goes the extra mile to find a cure and meets with Master Toji, a physician treating plague victims outside the palace. He gets a prescription and takes it back to his mother.
The Crown Prince begins to improve with the new treatment but it has to be kept a secret as bringing medicines from outside the palace or performing a treatment not prescribed in the royal records is illegal.
His unfortunate death
The Crown Prince recovers in time for the birth of his child and then attends lessons with his newly selected cohort, Prince Bogeom. However, at the lesson, the Crown Prince begins coughing up blood and his condition worsens.
As the calls to name a new heir grow, the reason for the Crown Prince’s regression of health is being investigated by the State Councillor under the orders of the King. Eventually, the Crown Prince dies and it becomes an investigation into his death.
Councillor Hwang tortures and interrogates Physician Kwon who was in charge of taking care of the Crown Prince but Kwon stands strong and doesn’t reveal anything.
Consort Hwang tells her father not to push through with the investigation as it would reveal something damaging to them. Consort Hwang shared a relationship with Physician Kwon and she gave him something that was only supposed to worsen the Crown Prince’s condition but ended up killing him.
Councillor Hwang temporarily stops the investigation but when he finds out about the medicine from outside, he aims to use it as an opportunity to get rid of the Queen.
A calculated accusation
The State Councillor accuses the Queen of bringing the medicine from outside in front of the King and coerces Physician Kwon to confirm the accusation. He also establishes that medicine is the reason behind the Crown Prince’s death.
Seongnam goes to the King and tells him that he was the one who brought the medicine and promises to bring in Master Toji and the herbalist to serve as witnesses.
He’s not able to locate Master Toji in time as the investigation carries on. The Queen throws a curveball and blames the Crown Princess.
She then meets the Queen Dowager and the State Councillor together and suggests demoting the Crown Princess and her children to commoners so that they lose their claim to the throne.
In reality, the Queen planned this with the Crown Princess to get her safely out of the palace. The Grand Heir was a target inside the palace and Queen wanted to keep her grandchildren safe.