As with much of the ancient Egyptian mythology, there are various versions of the story of Isis and Osiris, but basically it runs as follows.......
Osiris was an earthly ruler, who was popular with his subjects. His brother, Set, was jealous of this popularity and plotted against Osiris.
Set's plans to be rid of his brother started when he secretly obtained his brothers measurements and had a magnificent casket made to fit. This casket was in the form of a human shaped box.
Set then organised a large feast to which Osiris and a number of others (usually given as 72) were invited. At the height of the festivities Set produced the casket and announced that it would be given to whoever it fitted. All the guests tried the casket for size, but none fitted until finally Osiris stepped into the casket.
Set immediately slammed the lid closed and sealed the casket shut (with molten lead). The sealed coffin was then thrown into the Nile.
Isis was devastated at the loss of her husband and searched for the casket throughout Egypt and then overseas. She eventually eventually found it where it had come to rest in the roots of a massive tree.
Isis then returned the coffin to Egypt for a proper burial. For safe keeping she concealed it in the marshes beside the Nile.
Unfortunately for Isis Set found the casket while out hunting and was so enraged he chopped the body of Osiris into pieces, and scattered the parts throughout the land of Egypt.
Poor Isis had to then set out again looking for the parts of her husband. Eventually she found all the parts except one and reassembled Osiris and wrapped him in bandages. In some accounts Isis breathed life back into Osiris' body and it was then that Horus was conceived. This was a more magical event that it seems, considering the one part of Osiris Isis couldn't find.
The young Horus then went out to battle his uncle Set and to avenge his fathers death. After a series of fights detailed in 'The contendings of Horus and Set' neither god was able to secure an overall victory. Ultimately Osiris was declared king of the underworld,Horus king of the living, and Set ruler of the deserts as the god of chaos and evil.