Fortinbras is the Prince of Norway, a warlike young man with energy to spend.
He intends to revenge his father, Old Fortinbras, who was killed in a duel with Old Hamlet and thereby lost some Norwegian lands to Denmark. Restrained from doing so by his uncle the king of Norway, he invades Poland instead, to capture a useless little plot of land. This greatly impresses Hamlet. On his return, he passes through Denmark, and happens to arrive just as everyone dies. Since the entire royal family of Denmark is dead, he claims the throne on the basis of some old connection, Hamlet having already spoken in his favor.