Ep 31. The 520 Cenz Promise

The 520 Cenz Promise is the 31st episode of Fullmetal Alchemist: Brotherhood season 1, and it follows the story of Edward and Alphonse as they uncover the secrets of the Philosopher's Stone. The episode begins with the brothers' reunion with their old friend and teacher, Izumi Curtis, who is now living in the town of Dublith. Izumi informs the brothers that Scar, a dangerous alchemist killer, is on the loose and she fears that he may target them next.

As the brothers begin to investigate the whereabouts of Scar, they learn of a mysterious man known as the "Slicer" who is also after the Philosopher's Stone. The Slicer is rumored to be a member of a secret society called the Homunculi, who are said to possess extraordinary powers and abilities. Determined to uncover the truth behind the Homunculi and their connection to the Philosopher's Stone, Edward and Alphonse set out on a dangerous journey that takes them deep into the heart of the city.

Along the way, the brothers encounter a young girl named Lan Fan, who is a loyal servant of Prince Ling Yao of Xing. Lan Fan has been sent to Amestris by her master to seek out the Philosopher's Stone, which is believed to contain an incredible amount of power and could be used to save the prince's life. Despite their initial distrust of Lan Fan, Edward and Alphonse decide to help her in her quest, and the three forge an unlikely alliance.

As they continue their journey, Edward and Alphonse soon discover that the Philosopher's Stone lies within the walls of an abandoned laboratory, deep beneath the city. However, what they find there is far more shocking than they could have ever imagined. The laboratory is home to a group of human experiments, all of whom are children who have been forcibly turned into humanoid weapons. The children are all suffering from the same mysterious illness, which has left them weak and frail.

Disturbed by what they have seen, Edward and Alphonse vow to put an end to the cruel experiments and free the children from their suffering. They soon learn that the laboratory is owned by a wealthy businessman named Solf J. Kimblee, who has been using the children as pawns in his twisted experiments. Despite the danger, the brothers and Lan Fan hatch a plan to infiltrate Kimblee's mansion and rescue the children.

As the episode comes to a close, Edward, Alphonse, and Lan Fan successfully infiltrate Kimblee's mansion and succeed in rescuing the children. However, the victory comes at a great cost, and the three are forced to face the true horrors of the Philosopher's Stone. The 520 Cenz Promise is a heart-wrenching episode that explores the themes of sacrifice, loyalty, and the true cost of power.