In a museum overlooking a beachfront, a man named Lawrence sneaks through an elaborate art collection, and makes off with an ancient scroll.
The next day, Charlie Moss and Don Pringle arrive in California after finishing their university classes in Arizona. Charlie is bringing Don to visit Catalina Island, as Don has spent his entire life among the desert setting.
While on the boat, Don soon becomes enamored with a dark-haired girl staring off into the ocean. Don attempts to get to know her better, when Charlie takes him aside with some other teenagers, and they dance and listen to Little Richard sing a song on board.
Eventually, the ship arrives at Catalina Island. As Charlie disembarks, three girls immediately accost him a romantic fashion. Don watches the dark-haired girl leave in the arms of another boy. Lawrence departs and presents a cane to a man with a yachtsman hat. The two depart soon after when a man in a grey suit appears right behind Lawrence.
Lawrence and the man in the yachtsman hat board a boat in the Catalina Island harbor. The man in the hat is Arthur Duval, who has come to Catalina with his wife Anne and their son Tad. Though Tad doesn't know it, Arthur and Anne have hired Lawrence to steal the scroll. Their plan is to have Anne make a copy of the scroll, and pass it off as the original to a Greek man named Lakopolous. Once he has purchased the fake scroll, Lawrence will return the real one back to the museum he stole it from.
Arthur, Anne, and Lawrence's moment is spoiled when a young woman in a boat almost sideswipes their yacht. The young woman it turns out is Charlie's sister, Tina. After the close call, Tina meets up with Don and her brother, and the three then meet Arthur, Anne, and Lawrence, along with the Tad. Charlie invites Tad to join them on a scuba diving party, but Tad declines. Even Arthur tries to get Tad to go, but the young man suspects his father may be up to some crazy scheme on Catalina.
Afterward, Charlie, Don, and Tina go scuba diving not far from the Duval's yacht. However, trouble brews aboard the yacht when a man in a scuba suit comes aboard, claiming to come for the scroll for Mr Lakopolous. A scuffle ensues between Arthur and the man, and the scroll (stored in a watertight case) ends up falling into the water. The man attempts to escape, but Don and Charlie see what has happened, and try to subdue him. Their efforts are for naught, and the man gets away.
Don and Charlie board Arthur's yacht, as well as a member of the harbor patrol. Arthur claims the man was after his wallet, and the others accept this explanation. Once they leave, Lawrence voices his concern that they've lost the real scroll, but Arthur says that he's confident they can get it back before Lakopolous sends his divers to retrieve it.
After the excursion, Tina, Don, and Charlie go to a beach party along with Tad. However, the party atmosphere changes when the dark-haired girl Don saw on the boat appears, enchanting almost every other man on the beach. Tina then attempts to hook up with Tad to make Don jealous.
Soon after, everyone heads off the beach to another function, but Don stays behind. As he watches the water, he sees the dark-haired girl calling for help. It soon becomes apparent that she has lost her bikini top, and Don gives her his jacket to cover-up with. Upon thanking him, the girl explains that her name is Katrina Corelli.
Don attempts to get her to join the others, but she claims she's waiting for her boyfriend, Angelo. When she asks Don to help her, the two venture down the beach where they find Angelo. Angelo is upset to see Katrina with another man, and Don takes his leave. When Katrina asks Angelo what he has been doing, he dismisses her questions, claiming he should not have brought her to Catalina. This causes Katrina to leave him, and return to Don.
Don has returned to the others, but when Katrina shows up, Tina gets jealous and wanders off. As Don's talk turns to what happened to Angelo, Katrina reveals that he came to Catalina to help a man named Lakopolous, as well as being hired for his sea-diving abilities. The group then begins to wonder if Angelo may have attacked Tad on his yacht, when Tina is heard crying for help out in the water. However, instead of Don saving her (as she expected), the man with a hat who was trailing Lawrence off the boat does. This leads to every other girl on the beach growing jealous of Katrina, and marching off. Soon afterwards, the teens reconcile on a boat to the sound of a grooving tune.
Later on in the day, Arthur and Lawrence propose a scuba contest, promising a $100 reward if they can find the watertight cases at the bottom of the harbor. The gang goes for it, but during the 'contest', the previous diver along with Angelo shows up. Another scuffle breaks out, and the two divers get away, but not before Arthur finally admits to his son why they're really on Catalina.
Afterwards, Tad, Don, Charlie, and the harbor patrolman discuss a plan to catch Angelo and the other man who attempted to assault Arthur. Their plan soon leads to Angelo being arrested, but not before his accomplice takes off with a container that should contain the scroll. The chase finally ends when the man with the hat from before knocks the accomplice into the water, but manages to retrieve the scrollcase.
The man with the hat then identifies himself as Fingers O'Toole, who has been trailing Lawrence and Arthur. He attempts to arrest Arthur, but upon looking in the case, finds the scroll inside is gone. With no evidence against Arthur, Fingers loses his man.
The real scroll it seems has been safely taken away by Tad, who has managed to sneak back into the museum and return it. Back on the island, everyone else is dancing down by the docks to a swingin' tune.
- Tommy Kirk as Don Pringle
- Del Moore as Arthur Duval
- Peter Duryea as Tad Duval
- Robert Donner as Fingers O'Toole
- Ulla Strömstedt as Katrina Corelli
- Jim Begg as Lawrence
- Sue Casey as Anne Duval
- Lyle Waggoner as Angelo
- Mike Blodgett as Bob Draper
- James Almanzar as Sid
- Lee Deane as Dino Lakopolous
- Venita Wolf as Tina Moss
- Brian Cutler as Charlie Moss
- Peter Mamakos as Borman
- Musical highlights include Little Richard singing "Scuba Party" (seemingly while under the influence of some mind-altering substance) and The Cascades with "There's a New World Opening for Me." 
- Carol Connors (original name Annette Kleinbard), the singer of "Book of Love", had career highlights before and after this movie. She sang lead for the Phil Spector girl-group The Teddy Bears ("To Know Him Is To Love Him"), and later co-wrote the theme music for Rocky.
- Mary Wells, who sang the theme for the movie ("Never Steal Anything Wet") was one of the first breakout female stars of the Motown label. Her most famous song is the 1964 hit "My Guy".
- Both Never Steal Anything Wet and Scuba Party were titles planned for the film, before the makers decided on Catalina Caper.
- The movie was made by Executive Pictures Corporation, which had been formed by Bond Blackman and Jack Barlett. It started filming in September 1965. Tommy Kirk was signed to a four-picture contract of which this was to be the first (though he did not make any subsequent films with the company).
- Venita Wolf receives an "introducing" credit, though strictly speaking this was not her first movie appearance (she was a bikini girl in The Oscar from the previous year). The somewhat-common perception of such credits being career killers proved to be the case with her, as she only did a handful of TV appearances afterwards.
- The opening animated sequence was done by "Murakami Wolf" (later known as merely "Fred Wolf Films"). They previously did the opening sequence for the Woody Allen-dubbed Japanese film What's Up Tiger Lily! and would later be known for producing the 1987 Teenage Mutant Ninja Turtles animated series.
- A 1969 episode of the TV series Dragnet includes a stock footage shot of a movie theater marquee that shows Catalina Caper on a double bill with the Elvis Presley film Speedway (from 1968).