Sinbad and his pirate crew attempt to steal the magical, mystical "Book of Peace" while it travels to Syracuse, Sicily, protected by Proteus. Proteus was once Sinbad's best friend as a child and he tells him if it ever meant anything he can prove it. Sinbad tries to steal the book anyway, but is prevented when Cetus attacks the ship. The two work together to fight off Cetus and for a moment reaffirm their bond. Just when it seems the beast is defeated, Sinbad is dragged off the ship. Proteus goes to save Sinbad, but as he is royalty he is stopped by his crew. Drawn underwater by Cetus, Sinbad is saved by Eris, the beautiful Goddess of Discord, who offers him any boon he desires in exchange for the Book of Peace. Sinbad and his crew go to Syracuse to steal the Book; but leave without doing so. Anticipating this, Eris impersonates Sinbad and steals the Book. Sinbad is sentenced to be executed, whereupon Proteus sends Sinbad to retrieve the Book instead, placing himself as hostage, and Proteus' fiancée Marina goes to make sure that Sinbad succeeds. To prevent them from succeeding, Eris sends a group of mythical sirens, who entrance and seduce the men aboard Sinbad's ship with their hypnotic singing voices, but do not affect Marina, who pilots the ship to safety. She later sends a Roc to capture Marina, but it is killed by Sinbad.
After these and other incidents, Sinbad and Marina enter Eris' realm, where she reveals that her plan was to maneuver Proteus into Sinbad's place, leaving Syracuse without an heir, and agrees to surrender the Book of Peace only if Sinbad truthfully tells whether he will return to Syracuse to accept blame and be executed. She gives him her word that she will honour the deal making it unbreakable even for a god. When he answers that he will, Eris calls him a liar, and returns him and Marina to the mortal world. Sinbad is ashamed and he admits Eris is right and that he truly believes deep down he is a selfish liar.
In Syracuse, the time allotted to Sinbad has elapsed. Proteus readies himself to be beheaded, saddened by his friend's betrayal. At the last minute, however, Sinbad appears and takes his place. An enraged Eris appears suddenly and saves Sinbad by shattering the executioner's sword to pieces. She berates him for his actions. Sinbad, shocked, realizes that this was still part of her test and that he has beaten her by proving his answer to be true after all. Eris is furious but cannot go back on her word and so reluctantly gives the Book to Sinbad before vanishing.
With the Book restored to Syracuse, Sinbad and his crew leave Syracuse on another voyage, leaving Marina behind. Proteus sees that Marina has fallen deeply in love with Sinbad, so he releases her from their engagement and sends her to join Sinbad's ship.