Becoming His Own Grandfather

"For example, if you killed your grandfather, you'd cease to exist!"
―Hubert J. Farnsworth
"But existing is basically all I do!"
―Philip J. Fry I

The Planet Express crew had gone to observe a supernova up close. While the crew prepared for the event, Fry went to make popcorn. He ignores the warning label on the aluminum popcorn pan saying not to cook in a microwave, which he does. The microwave emitted radiation that collided with the wave from the supernova, causing the Planet Express crew to travel back in time to 1947.

After Bender breaks apart, Zoidberg was captured by the U.S. military while picking pieces of his torso. Philip I and Bender's head attempt to infiltrate Roswell, with the former disguised as a soldier. He runs into his grandfather, Enos Fry, engaged to Mildred. Acting on Hubert's warning, Philip I did his best to make sure that his grandfather was not killed- otherwise, he, in turn, would cease to exist. However, he became paranoid in his attempts to protect Enos, driving and locking him in a house in the middle of nowhere - a house that was, unbeknownst to Philip I - within range of a nuclear testing ground. Needless to say, Enos was killed in the resulting explosion, but for some reason, Philip I was still existing.

He then returned to comfort his soon-to-be grandmother, who began to make sexual advances on him, which made him uncomfortable. He then reasoned with himself that because he still exists, Mildred couldn't be his grandmother and had sex with her. The next day, Professor Farnsworth, Leela, and Bender knocked on the window, informing Philip I that because of his actions, he himself became his own grandfather: Philip I realized what he had done and freaked out. Because of the paradox, the Planet Express crew decided that they should get back to their own time, regardless of what it would do to change history. They stormed Roswell, rescuing Zoidberg during an autopsy, Bender's reconstructed body and stealing a microwave satellite dish to return to the future.

It was later explained by the Nibblonians that as a result of Philip I becoming his own grandfather, a genetic abnormality caused him to lack the delta brainwave, rendering him immune to the Brainspawn and telepathy in general.