When Macbeth ascended to the throne of Scotland after Duncan's murder, Banquo naturally started to wonder if his friend had something to do with the deed. Sensing Banquo's unease and wanting to prevent his prediction from coming true, Macbeth hired murderers to kill him and his son. Though Banquo was killed, Fleance escaped, and so the plan backfired.
Later that night, at the feast Macbeth was holding for his lords, Banquo's ghost appeared to haunt the man who had brought about his murder.
After that Macbeth did see the ghost of Banquo once more, following the line of kings (his descendants) that the witches had conjured up.