Thank God It's Doomsday is the nineteenth episode of the sixteenth season which first aired on the 8th May 2005. The episode shows Homer watch a film and being scared into it happening in real life and starts to think that he is seeing 'signs' of the apocalypse and predicts that the world will end in a week!
Bart and Lisa go to a barbershop labeled Da Buzz Club, but their haircuts go horribly awry, and they're forced to hide with Homer from school photographers from Springfield Elementary inside a movie theater. The theater is showing a Christian movie called "Left Below". After watching the movie, Homer thinks that the Rapture is coming and gets frightened. Marge and Lisa reassure him which comforts him into thinking that it won't come.
As Homer is driving the very next day, he thinks he sees the Devil, which is a man in an advertising costume. This thought process is reinforced by "the Devil" saying "follow me...uhhh" which was actually the man telling Homer to follow him into the restaurant he was advertising. Homer then sees blood falling from the sky, which is actually from an injured whale being carried by a helicopter. Because of seeing those two things, Homer convinces himself that the Rapture is coming. He tries to estimate the time and date, by using methods which don't make sense, and it thinks it will come on the 18th May at 3:15 p.m, which is a week away.
Homer goes around Springfield, telling everyone about this, but they don't believe him. He then says that the stars are going to fall from the sky. This actually happens, as there is a blimp accident at the "Krusty's Celebrity Salute to Specials". This makes everyone believe Homer, thinking he has actually foreseen this event. His believers follow him to the Springfield Mesa awaiting the apocalypse. However, 3:15 soon passes and nothing happens. This annoys everyone in town, including Moe, who sold his Tavern to a Japanese Sushi bar. Homer, upset, goes home and throws away all the books on how to predict the future, but realizes that there was thirteen people at the Last Supper, not twelve (as he used this as a way of predicting the Apocalypse). This means that the Apocalypse will happen late at night on May 19; not in the afternoon on 18 May. Homer tries to convince his family to help him warn everyone again, but they don't believe him. Homer, alone, makes his way back to the Springfield Mesa and when the hour passes, he thinks nothing is going to happen again. However, the next moment, he is naked and floating through space.
Homer is in heaven with a tourguide, which gives him some clothes, much to Homer's dismay. They fly pass a few heaven houses, where Homer sees a waterslide which can't be used for another year due to Leprachaun labour. Homer is told by the tour guide that all his wishes will come true in heaven. Homer wishes that the tour guide's head explodes, and it does.
Because no-one followed him to the Mesa, they are being tormented by the Devil. Homer asks to see his family on the big screen TV and wants to save them so he goes to see the Lord. The Lord refuses to help him because Earth is where Jesus suffered. Homer threatens the Lord, saying he has made a very big mistake and made a powerful enemy. Homer goes on a rampage around heaven to change the Lord's mind, but gets stopped by security. The Lord then agrees to undo the Rapture and turns back time. The Lord says "Deus ex machina". Homer wakes up from this dream at the Mesa and is reunited with his family. He goes to Moe's Tavern to find it has been restored, and that is Heaven to Homer.