|"Forward to the Past"|
The comic book Back to the Future 2 is an adaptation of this episode.
Traveling back in time to the Cretaceous Period with Jules and Verne, Doc Brown tests his latest invention - a device which effectively disintegrates objects - on a meteor headed for the Earth. Returning to the future, they find themselves in an alternate 1991, where humans don't exist and dinosaurs have developed a human-like civilization. Before they can be erased from existence, the Browns return to the prehistoric era and Doc reintegrates the meteor, restoring the correct timeline.
To test a new invention called the Proprietary Ultrasonic Subatomic Molecular Redistributor, Doc and the boys head back to prehistoric times so that they won't endanger anyone. Before they can run their test, a Tyrannosaurus gives chase and with the help of a friendly Pteranodon (whom Verne nicknames "Donnie"), they escape. At nighttime, they finally set up to run the test. Jules spots a shooting star, which is actually a large meteor, heading toward earth. Doc zaps it with his redistributor and destroys the meteor, thus saving Earth, but the car battery has been drained -- not by Doc's rewiring but by Verne watching a videotape.
For a moment, it looks like they are stranded, but Jules spots a grove of lemon trees, and Doc and Verne use copper, aluminum, and the lemons to generate 12 volts for the battery. After returning to 1991, they realize that the landscape has not changed, and that highly evolved dinosaurs have developed their own civilization. Having altered the timeline, Doc and the boys must leave before their own existence is erased. Doc realizes that they must restore the meteor and allow it to impact with the Earth, even though it means that the pteranodon that saved them will be headed toward extinction with the rest of the dinosaurs. When the group returns to present-day Hill Valley, Verne is broken-hearted about the death of Donnie, but takes some consolation in finding a little bird that resembles the pteranodon.
Doc: That meteor is on a collision course with Earth!
Verne: Cool! Can we stay and watch?
Jules: Certainly, until it explodes on impact and annihilates us.
Verne: I'll go wait in the car.
- "Out of the beaker, into the Bunsen burner! It appears we are invited to dinner-- as the main course!"
- —Doc, after Donnie the pteranodon flies off with him and his kids
- Jules: "Ow! What is the point of that brutality, sibling?"
- Verne: "I wanted to make sure we weren't dreaming."
- — Verne pinches Jules on the arm
- "Twelve thousand pounds at 5.8 cents per ounce, the standard rate for the aforementioned metallic element, computes to $121,360-- hardly a 'jillion'."
- —Jules, after hearing Verne's plans to make "a jillion dollars" from collecting aluminum foil gum wrappers
- Doc: "Jumping jigowatts! By preventing the extinction of the dinosaurs, we also prevented humans from developing! Your mother, Marty, Einstein... all the people we know and love don't even exist in this alternate world. And since our ancestors weren't born in this timeline, we can't live here either. By my calculations, we will completely dissipate in 12 minutes and 17 seconds."
- Verne: "No way! I'm too young to be erased from existence!"
- This episode grossly claims that the Cretaceous Period was in 3,000,000 BC. The period actually lasted from 145 to 65 million years ago, so it would have been long over by that date. Though dinosaurs did flourish in the Cretaceous Period, the largest reptiles by 3 million BC were crocodiles.
- In the episode, Jules spots a grove of lemon trees in Hill Valley during the Cretaceous Period. Fruits of the genus Citrus originated in tropical southeast Asia. Lemons were first introduced to the Americas in 1493. 
- However, an electrical charge can be generated from chemical reactions between a lemon, copper, and aluminum. In the live segment at the end of the episode, Doc Brown comments, "Sounds like something out of a cartoon, doesn't it? But it works." With the help of Bill Nye, Doc demonstrates how three lemons can generate enough voltage to power a digital clock.
Behind the scenes
- Forward to the Past is, perhaps, the most cerebral of the animated series episodes. In addition to the Brown family's return to an alternate "posthistoric" present, the show also explores the oddities of time travel. Marty can be heard telling Einstein a joke as the DeLorean departs, and after the timeline is repaired and the Browns return to their present, he can be heard telling the punchline (the Browns had returned two seconds after their departure). When the family returns to the Cretaceous period to fix their mistake, they arrive early enough to watch their younger selves departing. Finally, Doc has to answer his children's questions about the ethics of setting the meteor back on its collision course with Earth, and leaving a friend behind to die.
- The episode has some interesting surreal animation, including a moment where Doc thinks at a hyper-accelerated speed, and a chalkboard animation sequence.
- The "Cretaceous-Tertiary Extinction Event", which marked the beginning of the extinction of the dinosaurs, is believed to have occurred 65.5 million years ago.
- The meteor itself is believed to have been an asteroid about 6 miles in diameter. Clearly, the redistributed molecules of the meteor remain in outer space rather than being stored in the redistributor itself.
- The point of impact on Earth was at the location of modern-day Chicxulub on the Yucatan Peninsula in Mexico. The descending meteor, which the Brown family spotted about 4 minutes before impact, would have been visible from California.
- The odds against the Browns arriving at the hour of impact of a body from outer space would be, like the body itself, "astronomical".
- The dinosaur the Browns bumped into resembles Biff Tannen.
- Doc Brown Blackboard
- Electroglide boots
- Lemon battery
- Proprietary Ultrasonic Subatomic Molecular Redistributor
- Universal Linguistic Translator
New slang and phrases
- Galloping Galileo!
- I'll be a simian's uncle
- Okey monokey
- Out of the beaker into the Bunsen burner
- Ring my Alexander Graham Bell
- What in the constantly expanding and shrinking universe?
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