Barney Hendrickson is a mediocre movie-maker with no prospects, and his employer, Climactic Studios, is about to go out of business, particularly as the owner, L M Greenspan has systematically looted the corporation. In desperation, he drags Greenspan to see Professor Hewitt, who claims to have a time machine. Hewitt's idea of a demonstration is not Greenspan's, sending a bottle half a second into the past. Barney desperately takes over the machine, turns everything up to maximum and sends Greenspan's Cuban cigar three seconds into the future, burning out the circuitry in the process.
"L M" is convinced enough to finance a replacement, under cover of a mad scientist's plot device for a horror movie. Barney aims to make a historical movie in the past, with a minimal crew, a handful of actors, and all the other parts filled by extras recruited (hopefully for wampum and beads) from the local people. His first script is "Viking Columbus", about the founding of the Vinland settlement.
On the first trip, they capture a Viking named Ottar in Orkney of the 11th century. With Ottar as local guide and interpreter, paid in bottles of Jack Daniel's, they move an entire sound stage into the past and begin filming. Their stars are Ruf Hawk, a narcissistic he-man, and Slithey Tove, a sex goddess.
In the present the auditors from the bank are arriving in a few days, at which point they will discover the cooked books. Also, Prof. Hewitt explains that the time machine must return to the present, a moment after departing, before going to any other time. As a consequence, time used in the present day is lost forever. Barney thinks he can stay in the past as long as necessary, but first Ruf backs out and has to be returned. Then the time machine itself breaks down in the present day, taking hours to repair.
Improvising desperately, Barney casts Ottar in the lead and resolves to stay in the past until all is done. They film Ottar setting sail for Vinland with an actual colonising crew, then the movie people jump, via present day Hollywood, to Newfoundland a few months later, tracking Ottar by a radio beacon on his boat. While waiting, they have a nasty encounter with the natives, whom the Vikings will call skraelings.
Ottar hits the beach and digs in while the cameras roll, then the crew jumps forward a year to give the colonists time to build the settlement and stockade. They learn that Slithey, who had become smitten with Ottar, had been left behind by accident. Not only that, she has Ottar's baby son on her hip.
They continue filming, even during a mass attack by skraelings, which is repelled with tear gas and Viking axes. Shooting the final scenes, Barney is triumphant - and downcast. Even with all the film shot, he still has to do a soundtrack, dub the spoken lines, and edit the final cut. There is no time in the future to do this, and no equipment in the past. All seems lost.
Barney is all set to walk into Greenspan's office empty handed, when he gets a surprise visitor - himself. He takes in the film he has given himself, makes the film, and goes back in time to deliver it to himself.
With the movie a smash hit, Climactic's future is assured. Only one puzzle remains. The Vikings went to Vinland, but where did the real settlers land? The Norse sagas say that the expedition was led by Thorfinn Karlsefni, but there was no sign of him. Then Ottar, overhearing, reveals that his formal name is indeed Thorfinn Karlsefni. Shocked, Barney and Hewitt realize that the colony was founded because they decided to make a movie about founding the colony. Not only that, but the sagas mention another important figure, Bjarni Herjólfsson. Barney Hendrickson realizes: "They wrote a part for me!"