The script was rewritten by Guy Endore, then rewritten again by co-producer Frank King a week before filming in order to reduce production expenses; the film was then reedited prior to release. Haskin also shot some of the film's special effects sequences for a television pilot of the film for MGM Television, but no network picked it up.
The peaceful kingdom of Baristan is ruled by the evil El Kerim. He plans to capture his rival, Sindbad, who will soon return from a voyage to marry Princess Jana. The Princess convinces the magician Galgo to turn her into a Firebird, so that she may fly off to warn Sinbad of the trap set for him. She is able to do so just as Galgo is discovered by the guards, who take him to El Kerim. As Sindbad and his crew sail towards Baristan, the Princess/Firebird lands on the ship.
Before she can deliver the message, however, El Kerim has his guards transformed into giant human falcons, which manage to sink Sinbad's ship using large rocks. Sindbad and some of his crew survive the attack and carefully make their way ashore. Galgo stretches out his arm over an enormous distance in order to steal El Kerim's magic ring. El Kerim, however, wakes up in time and burns Galgo's hand.
Sindbad, posing as a petty thief, is arrested and must appear before the evil ruler. El Kerim is not fooled by the pretense and orders him to be executed. Sindbad breaks free and stabs him directly in the heart with a sword, but El Kerim cannot be killed. He orders Sindbad be put to death in the public arena the next day.
Sindbad must now do battle with "The Thing", a fearsome giant invisible creature. Fortunately, The Thing knocks over a torch in his pursuit, starting a large fire, inciting a mass exodus of the large crowd, which allows Sindbad to escape from the arena during the confusion.
Sindbad goes to Galgo and finally convinces the magician to reveal El Kerim's secret: the evil ruler had his vulnerable heart removed, and it is now kept safe in a distant bell tower, guarded by supernatural forces. In the meantime El Kerim insists that the Princess marry him, but when she finally refuses, he orders her to be executed with Sindbad when he is recaptured. Sindbad and his men must traverse a swamp of horror in order to reach the tower containing the evil heart. On the way, they encounter carnivorous vines, giant prehistoric crocodiles, volcanic lava pits, and killer whirlpools. Their numbers reduced, they finally arrive at the tower, where they dispatch a huge ogre with the multiple heads of a dragon.
Sindbad, with the aid of a hook, begins climbing the tower's immense bell rope. When he reaches the top, he discovers El Kerim's beating heart is encased in crystal. The heart is protected by a giant hand, which chases Sindbad until he throws the hook at the crystal, dislodging the evil heart, giving El Kerim a heart attack. Seeking to protect his vulnerable heart, El Kerim rapidly flies with Galgo in his winged chariot to the tower. Sindbad is about to impale the heart when El Kerim arrives. A fierce sword fight between them begins. Galgo steals the heart and tosses it over the side of the tower. As it falls to the ground, El Kerim faints, and falls from the tower's edge to his death. Later, the entire kingdom celebrates the marriage of Captain Sindbad and Princess Jana.
Outtakes of the film's "Dragon-Ogre" where used briefly in the film Natural Born Killers.
New York Times review, July 4, 1964
"Until about the last 20 minutes, it's strictly a broad mishmash of fantasy-comedy, spilling out over some lavishly gaudy sets of Old Arabia. As for plot, there's sinewy Sindbad (Guy Williams) trying to rescue a dead pan princess (Heidi Bruhl) from a wicked ruler (Pedro Armendariz), aided by a tippling, belching old magician".
"Throw in a tired "Scheherazade"-type of score, as Mr. Williams braves anything from crocodiles to a 12-headed monster (our count, anyway), and you have the kind of harmless trash some kids may tolerate".
"Yesterday, a cute little blonde in front of us took it all in stride, monsters included. She also perked up, leaning forward, for that final reel, when the picture slips from mediocrity into a wildly funny, eerie and casually beguiling adventure, not hard to take".
"One set, a garishly tangled swamp, is nifty; so is the final one, for a palace skirmish. The King Brothers produced it, in Munich, of all places. M.G.M. sponsors".Gold Key: Captain Sindbad (September 1963)