The Monster from the Lost Lagoon was actually an amphibian member of the alien Quon species who crash-landed on Earth an undetermined time before but needed time to repair his space ship. When human ships entered the lagoon, the creature reacted to their presence with fear and sank them all, which eventually brought him to the attention of the Fantastic Four. They had come to the Lost Lagoon for a vacation, but the Navy asked them to investigate the sinkings.
While the creature swam in the lagoon he ran across three of the four heroes in a borrowed Navy air-sea cruiser and reacted as he did before by sending a whale against their vehicle; though the cruiser was made by Tony Stark personally and survived the impact. The creature later watched as they returned to land and Mister Fantastic and the Invisible Girl embraced. Right afterwards, he swam back under the water and came up in an undersea cave where he swallowed a mysterious vial that changed him into the form of an air breathing, human being. He swam up out of the cave and into the Oceanarium, where he had disguised himself as a fish-handler.
The Fantastic Four appeared moments later and the creature anxiously wondered whether they had found him out. But they only wanted to see the Dolphin Show, and so he obliged them. Reed Richards, however, grew suspicious due to the closeness of the Lagoon to the Oceanarium, and probed his elastic arm into the tank to find a hidden cave. In order to protect his secret, the creature sent a pilot whale at Reed, smashing into his hand. Reed pulled it out and asked the creature if he would be their guide to the local caves.
The creature realized that Reed was suspicious, but he followed them to their air-sea cruiser and brought them to the lowest depths of the lagoon. There, he lured them into an undersea bog and smashed his way out of their vehicle to leave them to drown. But the Thing saved both the Human Torch and Mister Fantastic, using his increased lung capacity to swim them up and into the undersea caves the creature used to hide in. The creature, for he had finally turned back into his true form, faced the Thing. Ben tried to bring a stone wall down on him, but the creature moved too fast, and knocked the Thing from his feet with a single punch. Ben got back up and grappled with him until Johnny shot his flame around them, scaring the amphibian creature with its heat. He retreated back into his caves and to his then-repaired space ship.
He finished preparing his ship for the journey into space, loading it with excess plastic bags filled with water. Yet, the Thing had decided not to track him through the caves many tunnels, and simply smashed his way into the cavern where the creature hid with his space ship, along with his amphibian mate-who waited for him. When he saw what the cavern held, Reed realized the truth and stopped his teammates from attacking. The creature finished his packing and waved the three away from the front of his ship. Then he boarded it and launched himself into the deep waters, before turning upwards and blasting into the air and off the planet, then to parts unknown, while the Fantastic Four watched from the shore.