Tim and Bill are watching "Arthur C. Clarke's Mysterious World." Neither of them realizes that Arthur C. Clarke bears a suspicious resemblance to Graeme. Arthur C. Clarke discusses a series of mysterious objects and phenomena (alien artifact, alien connections to Stonehenge, raining frogs, teleportation, Loch Ness Monster, and Yeti). He dismisses them all as nonsense, even when they are staring at him in the face. He runs his show this way every week.
At the end, the announcer says it is the final of the series; it has been canceled because it has been proven that Arthur C. Clarke does not exist. Tim is incredulous at this and believes that Arthur C. Clarke does exist. When Bill and Graeme express skepticism at Arthur C. Clarke's existence (and Graeme furtively hides his Arthur C. Clarke wig), Tim sets out to prove his point.
Tim's quest for Arthur C. Clarke eventually leads to the Canadian Rocky Mountains, where Arthur C. Clarke has been sighted. Bill and Graeme tag along on the expedition. Once they set up camp, Bill reports sightings of a lot of paranormal phenomena but not Arthur C. Clarke, while Graeme reports nothing. Tim also reports a lot of huge footprints, apparently made by Bigfoot, while the others notice that his right leg is limping badly from all the walking he is doing.
Finally, Tim captures Arthur C. Clarke on film. The film is in the style of the Patterson-Gimlin film of Bigfoot. Graeme tries to block Tim from screening it. But Tim and Bill do see it, and then they finally notice the resemblance to Graeme. Graeme admits he is Arthur C. Clarke.
Then Graeme goes outside, where he is terrified to see huge Bigfoot prints on the ground. Nobody notices how odd it is that they all seem to be made by a right foot.
The expedition now turns to capturing Bigfoot, but pursuing the footprints gets no results. Then, during the night, Graeme and Bill notice how abnormally huge Tim's right foot has become. Tim then goes off sleepwalking, and Graeme and Bill realize that Tim's swollen foot is making the prints while he sleepwalks. They then see the funny side of it and burst out laughing. But Tim is not amused when he discovers this. He runs away in shame at being Bigfoot and the others laughing at him over his foot.
Tim goes living in the wild and tries to hide his ever-growing foot. But inevitably someone sees it sooner or later, and when they do, they always die laughing because it is so funny. This makes Tim a wanted man, with a huge price on his foot. Bill and Graeme set out to capture Tim and claim the bounty.
Meanwhile, Tim comes to accept and take pride in his foot, while making friends with the creatures of the wild. They help Tim to fight Bill and Graeme when they try to capture him. After a chase, Bill and Graeme end up with swollen right feet similar to Tim's, and it is their turn to be laughed at.