The dawn of Zinthar is at hand! The boys' discovery of a prehistoric relic spawns a monster that threatens to destroy South Park and the world. Help is on the way, however, in the form of an award-winning actor, a celebrated movie critic, and a rock superstar. But are they enough to overcome the terror of Mecha-Streisand?
During a fossil dig, Cartman discovers a mysterious stone triangle only to throw it away. Kyle picks it up instead, and after an archeologist identifies the writing on the triangle as Anasazi, Kyle appears on TV with the find. However, because of its newfound value, Cartman instantly wants the triangle back. After Cartman breaks into Kyle's room in the middle of the night to reclaim the artifact, Kyle tires of the situation and gives the triangle back to Cartman. Meanwhile, Leonard Maltin turns up in the town and asks Chef if he has seen Barbra Streisand anywhere. Chef is confused at this, but Maltin explains that if Streisand has seen the news report about the triangle, the boys are in great danger.
Later at the bus stop, Barbra Streisand lands in her helicopter and demands to know where "the Triangle of Zinthar" is. The boys don't know who she is nor do they want to give her the triangle, which aggravates her. Meanwhile, Chef is driving Maltin to Streisand's condo, while Maltin explains the situation—Streisand's "mother was a Jackal, and her father was an insurance salesman" and that she is an "evil, egotistical diabolical bitch" who wants to conquer the world with an ancient relic called "the Diamond of Pantheos." The diamond was split into two separate triangles and hidden at different points of the Earth, but Streisand found one of the pieces during the shooting of My Fair Lady. The other piece happens to be the triangle Cartman found, and according to Maltin if Streisand were to get her hands on that triangle she would become the evilest and dangerous creature to ever walk the Earth—Mecha-Streisand.
Later, a disguised Streisand finds the boys and tells them that the triangle is hers and that she will give them a monetary reward for it. Stan, Kyle, and Kenny are suspicious and reluctant to go with a stranger but Cartman drags them along, saying that stranger situations aren't bad if they include money. Streisand takes them to her condo in the mountains to complete the transaction. Once at her mansion home, Streisand ties up and tortures the boys with medieval torture devices and her singing until Cartman relents and gives up the triangle. She takes the triangle and combines it with the one she had unearthed years before, forming the Diamond of Pantheos. The Diamond transforms Streisand into Mecha-Streisand, a giant mechanical dinosaur-songstress that begins to completely destroy South Park. Shortly thereafter, Leonard Maltin and Chef find Streisand's home and free the boys, who fill them in on what had just occurred.
As Streisand goes about destroying the town, Leonard Maltin transforms into a giant robot, like Jet Jaguar or Ultraman, in hopes of defeating the evil Mecha-Streisand. Streisand proves too powerful, though, and Maltin is quickly defeated. Fleeing the battle, Kenny is strangled in a schoolyard tetherball. Sidney Poitier joins the battle and takes the form of a giant turtle much like Gamera. He, too, is no match for Mecha-Streisand, and South Park appears in utter peril until Robert Smith of The Cure arrives. He morphs into a giant moth, in the form of Mothra, and is told what to do through a wrist-watch, like Giant Robo. Discovering that Mecha-Streisand's weak spot is the nose, Smith is able to punch the Diamond of Pantheos out of her—using the Robot Punch—thus rendering her powerless. Smith is then able to mega-toss the weakened Mecha-Streisand into outer space, where she explodes. After the fighting subsides, the town cheers for Robert Smith, including Jesus, who hails Smith as a "savior". Robert Smith walks off into the sunset while Kyle shouts "Disintegration is the best album ever!"
The boys decide that the diamond is better off separated, as "no one should have that kind of power." However, the episode ends with it being re-discovered, and the boys cowering in the presence of a giant Ike Broflovski, Mecha-Ike.