In Mexico, Brock is engaged in a life-or-death struggle for a part for his '69 Charger, in a manly duel of . . . strip poker against a Mexican gangster. Despite the fact he's lost his knife, a naked Brock makes short work of the gangster's cadre of Mexican wrestlers by using his clothing and cigarette as deadly weapons.
Dr. Venture is lecturing at the Community University of Mexico but it's the Day of the Dead and the students are distracted. Dr. Venture and Brock prepare to leave but Brock is distracted by a Mexican prostitute and takes off. The boys show up and a monarch bug lands on Dr. Venture. It's actually a monitor device of . . . The Monarch! After disposing of a traitorous henchman, Monarch sends his promoted henchman Speedy to take a team.
Dr. Venture sends the boys off with some money and looks up his colleague Dr. Gueverra, who refuses him drugs for his "tripolar disease." Dean decides to fix H.E.L.P.eR. at an auto repair shop while Speedy drugs and kidnaps them. Brock gets their SOS and goes after them – a few 100 drugged darts and an automobile collision later, Brock finally goes down, along with Speedy who is trapped in his manly grip. The henchman are forced to leave them behind but take the boys to the Monarch's local lair. The Monarch is unhappy until he hears that Brock is incapacitated, but he can't get through to Dr. Venture to threaten him.
Meanwhile Dr. Venture wakes up and discovers his kidney has been stolen (again) and he's been left in a tub filled with ice. When Brock doesn't respond, H.E.L.P.eR. arrives (with his new Mexican paint job) to help him get back to the X-1, where through (relatively) quick thinking turns the robot into a dialysis machine, but needs a kidney from a compatible donor and finally looks up the boys. He homes in on Brock's wrist-radio signal and finds . . . a grave!
Meanwhile, a morose Monarch checks in on the boys repeatedly and reveals his origin – that his parents were killed in a plane crash in New Jersey's fabled Pine Barrens and he was adopted by monarchs as a young child. When they migrated south, he went to New York and claimed his inheritance, and is now in Mexico to visit with his adoptive butterfly parents (who died a number of years ago, not surprisingly). Monarch goes off to play father-figure to the Venture brothers while Dr. Venture mourns, only for Brock to emerge from his own grave.
The boys rush the Monarch while Dr. Venture and Brock close in on the hideout (after Brock dispose of a mythological chupacabra (someone has to do it). Brock takes the Chrysler directly into the hideout and disposes of a few hundred henchmen via automotive mayhem. The Monarch and Dr. Girlfriend flee and the boys play Rock Scissors Paper to see who will donate a kidney for their "best dad in the world." And at the end they both end up donating to their father's health.