Dr. Zin returns, and kidnaps a rocket specialist to get his hands on Dr. Quest's current project. The specialist's daughter Jessie escapes and goes to the Quests for help.

The Quest team go on a salvage operation to recover a World War II German submarine that mysteriously disappeared in 1945, but find a time machine on board that sweeps them back to 1945 and leave them no apparent way to return.

The Quests uncover a frozen creature, an orange-furred Viking guardian ape that they name Vikong ('Viking' + 'King Kong'). The creature is friendly, but the local villagers and greedy land developers are not, and the Quests help protect the creature.

The Quest team investigates when an oil-drilling operation uncovers a passageway to the center of the Earth. They go through and meet Hardrock, a creature of living rock, who joins their team.

The team travel to Tibet to investigate the disappearance of a scientist reputed to have fallen victim to clay warrior guardians angered by his disturbing their burial grounds.

A mad scientist creates a cloud-vehicle that can intercept any plane - the Quests must stop him.

A rivalry of sorts develops between Race and Dr. Quest when the latter builds a computer pilot and Race believes nothing can outfly a human. But during a test flight Race is kidnapped by a former rival from his past, a man badly injured and using robotics to replace his missing body parts (thus the name ""Skyborg""). Skyborg is out to steal Quest's computer guidance system on behalf of the foreign power that gave him his parts.

Delegates at a peace conference in India are plagued by hallucinations that set them against each other. Dr. Quest is called in to act as mediator while the others seek out Hadji's old mentor, only to find he's been kidnapped and his mental powers used to disrupt the conference.

A plant creature is responsible for the disappearance of a number of people in a nearby woods, and the Quests must deal with it before it spreads. Unfortunately they discover that the creature converts its victims into similar plant-creatures and soon an army of the vegetoids starts marching forth.

Dr. Quest comes to a London museum to determine the origins of an ancient crystal skull, and runs afoul of a group of Druids intent on unleashing its mystic powers.