When Horus grew to be a man, he challenged Set to the throne. A series of battles ensued but, to no surprise, Set didn't play fair and kept coming out the victor.
Eventually, Isis stepped in to help Horus. She set a trap for Set, but he begged her for his life and she let him go. This infuriated Horus. His rage was so strong that it even upset the other gods. In a final match, a boat race, it looked like Horus was going to be the victor. Infuriated, Set turned into a hippopotamus and attacked Horus' boat. Yet another fight ensued and their fellow gods declared the match a tie.
In the end, Osiris was consulted to see who should be king. Osiris declared that no man should take the throne through murderous ways, as Set had. In the end, Horus took his rightful place, while his father continued to rule the underworld.