Goemon

As a child, Ishikawa Goemon's (Ysuke Eguchi) entire family was assassinated for political reasons. His mother (Ryo) sent Goemon off moments before she was killed herself. Running away with his caretaker, they were attacked by bandits, but he was saved by Nobunaga Oda (Hashinosuké Nakamura) who offered him a chance to become stronger. Goemon followed Nobunaga and he was assigned to Hattori Hanz (Susumu Terajima) to train him in the ways of the shinobi (ninja) along with Saiz (Takao Osawa).After many years of training, Goemon was assigned to shadow Oda's niece, princess Chacha (Ryko Hirosue). They developed feelings for each other, but they couldn't be together and eventually she had to leave. Before going, Chacha gave Goemon her favourite fan to remember her by. As a gift for his service in protecting Chacha, Oda gave Goemon a double-bladed sword. Meanwhile, Oda generals Toyotomi Hideyoshi (Eiji Okuda) and Akechi Mitsuhide (Kazuaki Kiriya) conspired to kill Nobunaga. The two made a secret written covenant, placing their signatures onto a black scroll. Mitsuhide, not trusting Hideyoshi, had his men hide the contract. After assassinating Nobunaga, Hideyoshi betrayed and killed Mitsuhide, took credit for slaying Nobunaga's killer and became the next ruler of Japan. With Nobunaga dead, Saizo, ambitious to be elevated to Samurai status, opted to remain in service to Nobunaga, while Goemon chose to leave and be free. As a parting gift, Goemon broke his double-bladed sword in two, and gave one half to Saizo.Years later, Goemon became a master thief, and stole from the rich and gave to the poor. Sarutobi Sasuke (Gori), a wannabe bounty hunter, tried to arrest Goemon for the reward, but failed and became his servant instead. During a festival, Goemon infiltrated & robbed a treasure repository that containing a mysterious box with foreign writing on it. Ishida Mitsunari (Jun Kaname), a high-ranking samurai under Lord Hideyoshi, tried to retrieve this box to destroy it, but Goemon escaped with it, leaving behind only his calling card: a red sticker with the Japanese character 'Go' on it. Goemon, unaware of the value of the box, threw it away and distributed the gold to the people. A young pick-pocket, Koheita, picked up the mysterious box and kept it.The following day, Goemon learned of the box's value from Sasuke. Intrigued by the box, Goemon returned to the city to find it. Travelling around the slums in search of the box, Goemon heard a scream and finds Koheita and his mother, who had just been callously murdered by cruel and petty local samurai. Goemon saved Koheita and retrieved the box.
Saizo and his ninja squad, working for Mitsunari, appeared and confronted Goemon. Refusing to turn over the box, Goemon fled, pursued by the ninja. Goemon manages to escape all of his pursuers except Saizo, who remained close behind him. After dismounting Goemon, Saizo informed him that the box he carried was referred to as a "Pandora's Box" by foreigners, and the two engaged in a duel. After being caught and about to be killed by Saizo, Hattori Hanzo appeared and intervened, causing Saizo to withdraw.After returning to the city, Goemon examined the "empty" box and discovered a map to a mysterious treasure. Following the map, Goemon and Sasuke were led to the destroyed Buddha statue where Akechi Mitsuhide had hidden his contract with Hideyoshi. Goemon became angry once he learned that Hideyoshi was involved with Nobunaga's death. Hattori Hanzo then reappeared, under the command of Tokugawa Ieyasu (Masat Ibu). Hanzo offered his old student a bag of gold for the contract, which Goemon readily agrees to. Seeking vengeance for his murdered lord, Goemon infiltrated Lord Hideyoshi's castle, where he killed Lord Hideyoshi and discovered Princess Chacha. Startled by another person entering, Goemon hid himself in the ceiling of Chacha's room, losing Chacha's fan as he did so. To Goemon's surprise, the real Hideyoshi entered the room. Goemon had only killed his double. Suddenly, one of Lord Hideyoshi's guards discovered Goemon and shot him in the chest, blowing him out of the castle into the moat. Later, Goemon was secretly rescued and recovered by Saizo.
Goemon was then contacted by Hanzo, who took him to a waterfall to meet Chacha. The princess had come to bid farewell and return the fan as she had reluctantly agreed to become Hideyoshi's concubine.Afterwards, Lord Tokugawa arrived to ask Goemon to assassinate Hideyoshi in order to save Chacha. Elsewhere, Lord Mitsunari offered Saizo samurai status in return for killing Lord Hideyoshi. Goemon and Lord Tokugawa crafted a plan to assassinate Lord Hideyoshi during a celebration on his royal boat, but upon receiving the appropriate signal, Goemon changed his mind and did not complete the plan. Moments later, all of Lord Hideyoshi's escort ships were destroyed by explosives set by and Saizo and his team; they captured Lord Hideyoshi and tried to assassinate him by hanging him from the ship's mast with a steel chain. Lord Mitsunari, believing Lord Hideyoshi to be dead, then turned on Saizo and shot him. However, Lord Hideyoshi survived the assassination attempt, while Saizo survived the gunshot wound. Lord Hideyoshi, unaware of Lord Mitsunari's betrayal, interrogated Saizo, and threatened to kill Saizo's family unless he revealed who hired him.Meanwhile, Goemon tried to rescue Chacha, but as it became clear that Lord Hideyoshi was still alive, she refused to leave. Later, against Sasuske's warnings, Goemon rescued Saizo from prison. Unfortunately, this caused Lord Mitsunari to kill Saizo's wife and take his child as a hostage. Saizo was later recaptured and Lord Hideyoshi decreed he be executed during an elaborate public execution. Saizo was to be boiled to death if he didn't reveal his identity, and he loudly identified himself as Goemon. Goemon tried to stop the execution, but he could only watch in horror as Lord Hideyoshi kicked Saizo into the cauldron and then threw in the child. Sasuke, disillusioned and in shock over all of the carnage, abandoned Goemon.With the help of Saizo's surviving team, Goemon stormed through the palace and eventually reached Lord Hideyoshi. Goemon wanted to know why Lord Hideyoshi had betrayed Lord Nobunaga. Hideyoshi explained it was his insatiable hunger for power that had driven him. Goemon then killed Lord Hideyoshi. With Hideyoshi dead, Tokugawa and Mitsunari raised their own armies to decide who would be the next ruler of Japan. Tired of all the deaths and suffering, Goemon decided to intervene.Goemon charged into the battle wearing Lord Nobunaga's armor and carrying the reforged double-bladed sword. The armies were fearful and confused as they saw the crimson armor of Lord Nobunaga. Goemon fought through the armies until he confronted and killed Lord Mitsunari during a solar eclipse. Lord Mitsunari's army fled in fear. Goemon then charged through Lord Tokugawa's army, but Hattori Hanzo appeared and intercepted him mid-way. Goemon immobilized his former master by pinning Hanzo's foot to the ground with a broken sword blade. Approaching Lord Tokugawa, Goemon closed the distance seemingly intent on the kill, but was intercepted by Sasuke, who had joined Lord Tokugawa's army. Mortally wounded by Sasuke, Goemon staggered, and it was revealed that he was only holding Chacha's fan; instead of trying to kill Lord Tokugawa, Goemon only wanted him to promise that there would finally be peace. As Goemon tried to stumble to Chacha he fell to the ground. Dying, he admired the fireflies and stars while Chacha, alone, cried.This synopsis was taken from the English Wikipedia.

Question: Who decided to intervene?
Answer: Goemon; Goemon