(First Aired: August 21, 1966)
Screenplay by: Masahiro Yamada
Directed by: Samaji Nonagase
Fuji reports that a giant shark has been found dead in Yokohama Bay, clearly attacked by something; not to mention, many of the boats in Yokohama have been sunken by something lately. This also reminds her that Hoshino and his friends are at Yokohama Bay as they speak. Sure enough, the boy and two of his friends are watching a South American ship unload cacao beans. One of the kids sees something enormous surging through the water before submerging. Then Hoshino spots a shady-looking man wandering through the docks, so the trio decide to check it out. The kids lose the mystery man, so they decide to call Science Patrol about the monster instead. At the phone booth, Hoshino sees a Wanted poster of the man the notorious jewel smuggler Diamond-Kick. The kids instead report their sighting of Diamond-Kick to Science Patrol, who tell them thats a case for the police before hanging up.
While Science Patrol continues discussing about the mystery ship sinkings, Hoshino and his friends argue whether they should hunt the monster or tail the smuggler first. Thats when a sea captain bumps into them. Amused by their debate, the old seadog tells them about the story of the sea monster Gesura. Normally, an amphibious South American lizard with a penchant for sneaking onto cacao bean cargoes, Gesura was hatched from an egg in the polluted waters of Tokyo Bay, becoming a monster that ate cacao beans shipments. Then the ocean bubbles behind the group and out bursts a lizard-like monster with a prickly back and fins. The sea captain immediately identifies it as Gesura, whom the children try to get a closer look at. Thankfully, some sailors stop the kids while Gesura wrecks a docked ship.
Science Patrol receives a call about the sea monsters appearance and heads in. However, Gesura has already left. Meanwhile, Hoshino and his friends decide to visit the cacao bean warehouse and perhaps find a small Gesura. There, Diamond-Kick and his goon are cutting cacao bean bags open to try find diamonds they had smuggled from South America. The two overhear the kids coming in and kidnap them. Back with Science Patrol, the team is talking to the head of the docks about Gesura. Because another ship with cacao beans is coming into the harbor, they decide to bait Gesura by using cacao beans from the warehouse, creating a safe passage. At the warehouse, they stumble across the bag of smuggled diamonds inside one of the cacao sacks as well as a shoe belonging to one of the children. Science Patrol realizes Hoshino and his friends have bumped into Diamond-Kick. However, Gesura pops out of the water to wreck a ship, leading Science Patrol to hunt in the Jet V-TOL. Arashi, Hayata, and Fuji find the monster from the sky, while Muramatsu and Ide learn that Gesuras spines are poisonous. The latter group receives a report from the former that Gesura is swimming towards the cacao bean warehouse.
At the warehouse, Hoshino and his friends have successfully picked the lock of their backroom prison in Diamond-Kicks hideout. The smugglers notice the kids though, and a chase ensues. As for Science Patrol, they have gathered at the warehouse in time to hear the children crying for help and see Gesura shambling out of the sea. Diamond-Kick and his goon also spy the monster heading for them. The terrified criminals shoot Gesura, while the children make a sneaky getaway. Gesura flies into a rage, destroying various nearby structures, while Science Patrol finds Hoshino and his friends and takes them to safety. Hayata enters the warehouse to capture Diamond-Kick, but Gesura smashes the building to pieces. Hayata, pinned under the rubble, takes out the Beta Capsule and becomes Ultraman!
Ultraman arrives just in time to take Science Patrol and the children away from Gesuras massive foot; its especially helpful since Science Patrol firing has turned the monsters attention to them. Ultraman then battles Gesura, but not even he is immune to the effects of Gesuras venomous spines. Ultraman finally manages to grievously wound Gesura by ripping off its fin, forcing the sea monster to slowly slink back into the sea where it dies. With that, Ultraman leaves. As the kids shout goodbye to Ultraman, Hayata returns to the relief of his teammates. He even has Diamond-Kick and his goon, both injured by Gesuras rampage, in tow! Soon, the criminals are sent to prison, giving Science Patrol and the children the time to go to HQ and enjoy some chocolate.