Land of the Lost details the adventures of the Marshall family (father Rick, and his children Will and Holly) who are trapped in an alternate universe inhabited by dinosaurs, a primate-type people called Pakuni, and devolved, aggressive humanoid/lizard creatures called Sleestak.
The episode storylines focus on the family's efforts to survive and find a way back to their own world, but the exploration of the exotic inhabitants of the Land of the Lost is also an ongoing part of the story.
A number of well-respected writers in the science fiction field contributed scripts to the series (mostly in the first and second seasons), including Larry Niven, Theodore Sturgeon, Ben Bova, and Norman Spinrad, and a number of people involved with Star Trek, such as Dorothy "D.C." Fontana, Walter Koenig and David Gerrold.
The Marshalls are brought to the mysterious world by means of a dimensional portal, a device used frequently throughout the series and a major part of its internal mythology. This portal opens when they are swept down a gigantic 1,000-foot waterfall.
We later learn in "Circle", which explains the time paradox, that this portal is actually opened by Rick Marshall himself, while in Enik's cave, as a way for the current Marshalls to return to Earth, resolving the paradox and allowing Enik to also return to his time.
Outfitted only for a short camping trip, the resourceful family takes shelter in a natural cave and improvises the provisions and tools that they need to survive. Their most common and dangerous encounters are with dinosaurs, particularly a Tyrannosaurus rex they nickname "Grumpy" who frequents the location of their cave.
They also do battle with the hostile Sleestak (lizard-men) and "cave men" called Pakuni (one of whom, Cha-Ka, they befriend), as well as a variety of dangerous creatures, mysterious technology, and strange geography.
The main goal of the three is to find a way to return home.
At the start of the third season Rick Marshall is accidentally returned to Earth alone, leaving his children behind, and is replaced by his brother Jack. Spencer Milligan's absence was explained by having Rick Marshall disappear after he was trying to use one of the pylons to get home, and that Jack had stumbled upon his niece and nephew after he embarked on a search of his own to find them.
Though the term "time doorway" is used throughout the series, Land of the Lost is not meant to portray an era in Earth's history, but rather an enigmatic zone whose place and time are unknown.
The original creators of these time portals were thought to be the ancestors of the Sleestak, called Altrusians, though later episodes raised some questions about this.
Many aspects of the Land of the Lost, including the time doorways and environmental processes, were controlled by the Pylons, metallic obelisk-shaped booths that were larger on the inside than the outside and housed matrix tables — stone tables studded with a grid of colored crystals. Uncontrolled time doorways result in the arrival of a variety of visitors and castaways in the Land.