It is the only Flintstones production by Cartoon Network Studios after Hanna-Barbera was absorbed into Warner Bros. Animation on March 12, 2001 (Warner Bros. later re-acquired Cartoon Network Studios from Turner Broadcasting System, reuniting Cartoon Network Studios with Warner Bros. Animation, on March 4, 2019). In format, the film was intended to emulate the first two seasons of the 1960s series, distinctly more mature and aimed at older audiences than later seasons, and therefore chose to focus more on the relationships between the original core cast of Fred, Barney, Wilma and Betty. Set before Pebbles' birth, the Rubbles are prompted to take Fred and Wilma to Rockapulco for their anniversary vacation, in an attempt to rekindle their troubled marriage. However, while there, Fred and Wilma both find themselves tempted in other directions.
Since its original broadcast, On the Rocks has not been released on home video, though bootleg copies exist via various sources.
Plot
Fred and Wilma Flintstone visit a family therapist to try to fix their faltering marriage. Wilma is growing tired of Fred's attitude, especially while Barney and Betty are enjoying a happy life well into their marriage. Their session ends with a physical altercation between the two. On Fred and Wilma's anniversary, which they both forgot, the Rubbles arrange a trip to Rockapulco in an attempt to save the Flintstones' marriage.
Shortly after their arrival, a thief, Xavier, steals a diamond from a jewelry store and is chased by the guard into the same hotel the Flintstones and Rubbles are staying at. In the ensuing chaos, Xavier's bag is switched with Wilma's, and that's when he immediately begins plotting to get the diamond back. At first, things do not improve between Fred and Wilma, to the point that Wilma lashes out at Fred and nearly decides to divorce him, but when she stumbles across the diamond in her suitcase and, assuming that Fred bought it as a surprise present, she quickly makes up with him. Capitalizing on the circumstances, Fred goes along with the charade, but finds that their newfound passion is short-lived, as Fred's demeanor slowly puts Wilma off again. While spying on Wilma, Xavier notices this and masquerades as a suave Englishman in order to seduce Wilma by inviting her to dinner. Wilma accepts the invitation and spends time with Xavier.
Fred, feeling guilty, decides to make it up to Wilma, but catches her from afar with Xavier and is heartbroken, and he starts to drink himself silly while speaking with another attractive woman at the bar. Wilma rebuffs Xavier's advances out of loyalty to Fred, but changes her mind when she sees him with the lady. While dancing, however, Xavier reveals his true intentions and attempts to take the diamond from Wilma, who was wearing it as a necklace. A chase ensues throughout the ballroom with Fred, Barney and Xavier each trying to get the diamond, but fails when it eventually falls into Wilma's hands, prompting Xavier to abduct her and flee in his car. The ensuing car chase eventually leads to a bridge above a volcano, where Xavier threatens to kill Wilma if she does not hand the diamond over. Fred appears and gives a passionate speech about how he has not realized until now that even though he was not rich enough to buy the diamond, he is still the richest man in the world just by having Wilma as his wife.
Fred tries to attack him, but Xavier punches him unconscious. Wilma beats up Xavier in retaliation and ends up getting him arrested by the same lady who Fred spoke with at the bar earlier, who is revealed to have been a policewoman on Xavier's trail. With their marriage restored, Fred and Wilma enjoy the rest of their trip, while Barney and Betty begin to bicker about their own marriage after seeing the passion Fred and Wilma ultimately displayed for each other. Over the end credits, Dino, who was assigned by Fred to guard their home, is revealed to have made a complete mess and left the home in the hands of his friends before leaving on his own trip.
