The film begins with a couple of submarines going down to the wreck of the Thumbtanic. They find a picture of a naked woman and it is put on the news. The woman, Geranium (now old), picks up the phone without dialing and says that she's the woman then falls asleep. The news reporters arrive and the news anchorman verbally abuses Geranium, only causing her to fall asleep again, falling on the ground. She gets up by herself and then she tells her story of Thumbtanic.
In 1912, when old people were really young, over 2,000 thumbs are boarding the Thumbtanic. A young Geranium arrives in the 'sex scene' car and boards the ship. Jake, a strapping young thumb, thumb wrestles with a big guy and wins a ticket on the Thumbtanic. He tries to get on, but is stopped for a lice check. When he says he has no lice, the crewmen give him some, but he shakes them away. As the ship is sailing Geranium complains to a maid about being rich and wanting to be poor. The maide tells her she's crazy and his hit by a door. The captain asks Geranium if he wants to see his wheelhouse, and taking this to mean he wants to have sex with her, Geranium punches him. The captain insists he just means the ship's wheelhouse, and Geranium then agrees to go with him. There, she meets the ship's designer, who optimistically informs her that the ship has two lifeboats. One is intended to hold 24 of the "key nautical crew" and the other will hold everyone else. Geranium points out that the ship has over 2,000 thumbs on board, puzzling the designer, who had not noticed this before.
Jake sees Geranium and he says he likes the big fat thumbs and Geranium hopes that he likes thumbs that are big. Then Jake and many other passengers scream messages while standing on the bow of the ship. An important passenger, Mr. Prickle, visits the captain and demands that the ship go faster. The captain responds by ordering the engine room to "increase speed beyond reason". That night Jake saves Geranium, persuading her not to commit suicide as she had been intending to do. Stevine Odion sings "My Heart is a Thumb" while Jake and Geranium spend time together and then have sex in the sex scene car.
Then the Thumbtanic hits an iceberg and there is major flooding. Jake and Geranium exit the car and head onto the now tilted deck. A senior crewman attempts to calm the passengers, but the captain suddenly screams "We're all going to die!" causing mass panic. Mr. Prickle appears, loudly demanding that the ship sink faster. He leaves, declaring his intention to die the fastest, heading for the low end of the Thumbtanic. The captain consuls the ship's designer, who calmly suggests such solutions as a "heating fan the size of Africa" to dry up the ocean. He brings out a blueprint and begins discussing it, but the captain interrupts "This isn't useful; this is a monkey." The designer realizes it is, but then notes there is an escape portal in the tail of the monkey. The captain panics again, running off screaming, while the designer continues to study the monkey. On deck, a man sells babies, then sex changes (with an axe) to male passengers. A giant tarantula crawls on the ship, but Jake shoots it back into the sea with a laser from "Thumb Wars." The ship then goes up to a 45 degree angle and then goes backwards and has the bow stick in the air, then it goes back to the stern in the air.
A rocket is fired off, just before the Thumbtanic breaks in half and sinks. One thumb, observing in the water, notes that recreating the event on film would be extremely expensive. The captain, still on the bridge, suddenly realizes that staying on board after the ship went underwater was a bad idea.
Geranium and Jake bicker over who gets to stay on a floating table, shoving one another off repeatedly. They then see a lifeboat, but it is full to capacity with the designer and 23 other people in it. Jake gets an idea and all 2,000 passengers except the captain, Mr. Prickle and a man crushed by a piano are rescued by piling themselves on each other in the other lifeboat. The old Geranium adds that there were plenty of snacks to go around too, but she and Jake broke up the next day. She then falls asleep again and the anchorman steals the Blue Diamond Geranium had and gives it to the famous one-eyed thumb who was also on the Thumbtanic when it sank. A series of pictures show the one-eyed thumb enjoying his newfound wealth. The credits then play while Stevine Odion sings "My Heart is a Thumb" again.
A reference to Thumb Wars appears while everyone is talking about the ship getting hit by an iceberg. The first person to speak is a character from Thumb Wars who says, "We've been hit!"
As with other Thumb films, a one-eyed thumb appears in the film three times (the only time it appears three times in the films). The first time happens while everyone is talking about the ship. The next is when the ship is tilting. It finishes the film by selling the diamond in the picture of Geranium. On the DVD menu, the one-eyed thumb appears as a prank. If the viewer browses the DVD menu long enough, the one-eyed thumb appears as he does in the feature before shutting the DVD player down.
The film was released in 2002. It was also released in the "Thumb Wars/Thumbtanic Thumb Double Feature," along with Thumb Wars. Both were released on VHS and DVD in 2005. In 2005, it was released as a UMD Video on the PlayStation Portable system.