Fry, Leela, Bender, The Professor, and Dr. Zoidberg are watching a star that is about to turn into a supernova. Fry goes into the ship's kitchen to make some popcorn, but puts metal in the microwave. This energy combines with the energy produced by the supernova and it punches a hole in space that the ship is sucked into. The wormhole leads the ship into the past as they come out of the hole on July 9th, 1947, unknownst to the crew.
When Leela tries to land the ship, a layer of o-zone that wasn't there in 3002, and no global positioning signal cause the ship to crash on the desert outskirts of Roswell, New Mexico. Bender decided not to put on his seatbelt so he ended up flying out the ship's window on impact. When the gang went out to find Bender they found his body is pieces, although his head was still intact. Leela then ordered Dr. Zoidberg to pick up the pieces while everyone else went back to the ship. Soon, the US government went out to the desert to investigate the crash. They believe that the pieces of Bender's body are the remains of an alien spacecraft that Dr. Zoidberg was piloting. Luckily for the crew, the Planet Express ship was well hidden behind a large cliff of rocks.
The next day, the Professor realizes that they have gone through time and that the wormhole is closing fast. The only way for them to get back to 3002, is to find a microwave to put metal in to cause the same effect as before. Leela and the Professor say that they will go into town to find a microwave (not realizing that microwaves didn't exist in 1947) and Fry says he'll visit his grandfather on the Roswell army base. The professor bans it saying that if he does it could jeopardize his existence. Fry then changes his mission and goes out to protect his grandfather instead.
Meanwhile, the government is trying to figure out Dr. Zoidberg who isn't helping much with their study eventually leading them to call President Truman in who says that they need to do an alien autopsy because they can't get through to Dr. Zoidberg.
On the army base, Fry goes disguised as a soldier with Bender's head. They meet Enos Fry, Fry's grandfather, and Fry goes to great lengths to avoid getting Enos killed, but inadvertently locks him up in a house that is on a nuclear testing ground and he is killed in a nuclear test. The Professor and Leela in the meantime go to a local store incognito to try and find a microwave. The search is a failure. They go to a local restaurant to have lunch when Leela sees out the window that the army base has a microwave radar dish and that they could steal it and use it. The Professor won't allow it though, saying that it ill alters history and that they must preserve the past. That's when Fry comes in and tells them the bad news about killing his grandfather, yet, for some reason he hasn't disappeared from existence yet. Mildred, who worked at the restaurant and was Enos's fiancée, received the news via a phone call and was upset. Fry offered to walk her home and comfort her.
Mildred finds too much comfort in Fry and they end up spending the night together. The next day, the Professor, Leela, and Bender find Fry and tell him that he is crazy and that he slept with his Grandma. Fry assures them that she isn't his grandmother, but the Professor shows him that she is. Fry says he doesn't know who his Grandfather is and the Professor says that Fry had become his own grandfather putting Fry into a panic.
After getting back to the ship, the Professor says that they are going to steal the dish. Fry says that will change history, but the Professor says, "Oh, a lesson in not changing history from Mr. I'm-My-Own-Grandpa," then he adds, "Let's get the hell out of here, screw history."
In the highlight of this great episode, the crew then steals the radar dish, gets Dr. Zoidberg back, as well as Bender's body. On the way to outer space though, Bender's head falls out of the ship and is left behind. After getting back to 3002, the crew revisits Roswell and manages to find the head and everything then goes back to normal.