Naive and gullible, Peter gives his guitar to a scam artist in exchange for a treasure map.
- Peter is playing an acoustic guitar when a shady type appears out of an alley, identifying himself as Leonard Sheldon; he offers to sell Peter the city of San Diego or a map to a buried treasure on an island well off the California coast. Peter gullibly trades his guitar (which Sheldon tries to sell to Mike) for the map.
Peter later joins Mike, Micky, and Davy on their regular job working in the backlot of Columbia Pictures. They are naturally skeptical of Peter's map but decide to go on this seemingly foolhardy treasure hunt anyway. First though, Davy, portraying a British grenadier from the Revolutionary War, tries to sail with a rowboat overloaded with supplies despite the protestations of his pals; naturally he doesn't get anywhere.
Upon arriving on the island, lush with jungle foliage, the boys quickly find it isn't deserted, as a monstor's throaty roar in response to Micky's query as well as a plastic footprint found by Peter demonstrate. They are as yet unaware of the hut owned by cranky treasure hunter Major Pshaw and his man Friday, named Thursday, who enjoys watching The Monkees television series yet is always wondering aloud who the writer is. A tripwire awakens Pshaw and he and Thursday vow to "catch those blackguards" after ten years looking for buried treasure. When a swarm of mosquitos goes after the boys - and Micky's experiment with insect spray proves humorously ineffective - they find a net, which is attached to a crane used by Pshaw and Thursday.
Thus captured, the Monkees persuade Pshaw to give them a head start, so they make a break for their boat - except Pshaw has hidden it. Peter breaks down at getting his friends in trouble, though as Micky points out his crying can be effective as it was for Barbara Stanwyck, so they continue their attempt at escape, running into a somewhat mad Doctor named Schwarztkov before being set upon by elderly jungle jim Kimba, former movie star whose wife ran off with a casting director and whose child costar now lives in Argentina. Kimba tries to help them escape but himself needs rescue from quicksand; his attempt at summoning jungle friends merely gives The Monkees some extra pets and his attempt on a vine proves his swinging days are over.
Finally they run into Thursday, whom Davy recognizes from Stewart Granger movies. Thursday vows to help them get their boat and escape - and he earns the boys' trust because he possesses the weapon - but first must transition between scenes to Pshaw's hut. That doesn't work as Pshaw outsmarts them all - he vows punishments that run the gamut of a warm bath for Davy, fingernail cleaning for Peter, a rendezvous for Mike with his aunts, and a less-than-harsh lashing of Micky via a plastic tongue, but when Peter learns he is looking for buried treasure, he gives him the map - and they all find the treasure, except it isn't what they expected.