The play begins with a ghost talking to the audience.
Yeah, you read that right. But wait, it get's even weirder, because the ghost is accompanied by a dude from Hell called, "Revenge." And that's not just his name—he is Revenge personified. The ghost tells us that he is Don Andrea, a Spanish courtier who was recently killed in a war with Portugal.
Andrea's death is of a suspicious nature, so the King and Queen of Hell send him back to the living to get revenge with the help of his new friend, um, Revenge. These characters—and we do mean characters—will serve as Chorus for the rest of the play.
It turns out that Balthazar, a creepy-stalker-type-guy who is also the heir apparent to the Portuguese throne, killed Andrea. Even though Balthazar killed Andrea in war, he did so in a very dishonorable way (a cheap shot, if you will), hence the need for revenge. Andrea was lucky, however, to have a best friend in Horatio, who rallied to kick some major Portuguese butt in the process of capturing Balthazar. Because of his valor, Spain wins the war and Horatio greatly impresses the King of Spain.
As Horatio presents his prisoner to the King, this guy Lorenzo tries to take credit for capturing Balthazar, which is of course completely bogus. But because Lorenzo is the King's uncle he is given part of the reward, which includes taking good care of Balthazar until Portugal pays a ransom to get him back. Lorenzo and Balthazar become best buddies and ultimately partners in crime.
There is also a subplot about two Portuguese courtiers, in which one accuses the other of murdering Balthazar. We of course know Balthazar is alive and free to terrorize Spanish ladies. But the Viceroy of Spain is an emotional guy who rashly sentences an innocent man to death after wrongly assuming Balthazar is dead. This all sorts out in the nick of time, and the false accuser is immediately tortured and executed. 16th-century Portugal was not known for due process.
Back to the main plot: when Horatio returns from war he pledges to serve Andrea's mourning girlfriend, Bel-Imperia. Horatio tells Bel-Imperia about how Balthazar unfairly killed Andrea, which obviously enrages her. She then determines to love Horatio in part because she knows that's one way to spite Balthazar (Andrea's ghost is totally cool with this). To further complicate matters, Balthazar falls immediately in love with Bel-Imperia, who rebuffs the prince's slimy advances at every turn.
Lorenzo then gets creepily involved with hooking Balthazar up with his sister. He first pays her servant, Pedringano, to find out if Bel-Imperia has another lover. When he finds out that other lover is Horatio, he then pays Pedringano and another accomplice to hang Horatio from a tree and brutally stab him to death as Bel-Imperia watches.
Horatio's father, Hieronimo, is the Knight Marshal of Spain, which means he judges and condemns criminals. When he finds Horatio's bloody body hanging in his garden, he vows to revenge his son's death. Lorenzo has Rapunzled Bel-Imperia away in a tower or something, but she does manage to get a letter (written in her own blood) to Hieronimo informing him that Lorenzo and Balthazar are responsible for Horatio's murder. Hieronimo now knows who his targets are, but he needs more proof and the right opportunity to kill them—especially because they're both important royal types.
In the meantime, The King of Spain and the Duke of Castile (Bel-Imperia's father) have arranged for Balthazar to marry Bel-Imperia. They believe that doing so will secure a peaceful relationship between Spain and Portugal. As the proposed wedding approaches, Lorenzo begins to worry that his henchmen will betray him, so he comes up with a masterful plot to do away with both Pedringano and Serberine.
As Hieronimo's revenge takes longer than expected, consequences begin to mount: the ghost of Andrea becomes impatient, Isabella (Horatio's mother) goes mad and kills herself, and Hieronimo appears to be completely off his rocker. He tries to bring formal accusations to the King, but Lorenzo blocks his access at every turn.
Finally, Hieronimo decides that he has no recourse but to take the law into his own hands. He gets his opportunity when asked to produce a play in celebration of Bel-Imperia's wedding. Hieronimo was apparently a bit of a poet in his youth, so he drafts the play and asks Lorenzo and Balthazar to play leading roles. The evil twosome haltingly accept their roles, even though the characters they are asked get murdered in the play. Under the cover of theatrical action, Hieronimo kills Lorenzo, after which Bel-Imperia stabs Balthazar and then takes her own life.
To prevent himself from telling more of his story under torture, Hieronimo bites off his own tongue. The King orders that a pen be brought so that the tongueless avenger may be forced to write his story, but Hieronimo randomly stabs the Duke of Castile with the pen before using it to take his own life. Andrea and Revenge are greatly pleased with the performance, after which they assign suitable punishments and rewards for all the dead.