Graeme falls through the floor of their office and lands in a cave far below. Tim and Bill follow him down and there they find a Tyrannosaurus Rex's jaws, and accidentally lock themselves inside. They are forced to live inside the stomach forever and ever, until they suddenly think of crying for help through the T-Rex's vocal cords. This wakes up the T-Rex and as he yawns, they get out of his mouth and they climb up, only to see the T-Rex eat their house.
The goodies fall through a hole in their floor and find a T-Rex below.
- It's the weekend, and the boys each have their own ideas on how to spend their two days of rest and relaxation. Graeme curls up in a corner with a book, while Bill sets off a noisy contraption that will help him practice his saxophone playing, drumming, football skills, and bird watching skills all at once. Tim was about to put on some music so he could practice his tap dancing (throwing in a little ping-pong as well, to make it interesting) and is annoyed by Bill's racket, so he breaks Bill's contraption to silence it. Bill retaliates by making Tim's radio short-circuit and stomping on his toes so he can't dance. Both sulk for a few moments, then realize they are bored, so they call on Graeme for help.
Graeme is annoyed that they bothered him while he was trying to read. He tries to get them interested in his new "patent compendium of home entertainments", which converts - with a bit of moving of flaps and levers - to a TV, gramophone, keyboard, and washing machine in turn. Tim and Bill just make fun of it, and end up making too much noise again for Graeme's liking.
Graeme admits he's gone off all those mechanical forms of entertainment, too. He's currently into studying paleontology and archaeology. He explains he's been reading up on the subject, and shows them a map he's made of fossil layers he believes are under London. Tim and Bill find the interest silly and mock him through his dissertation whenever they get the chance. Graeme, however, is undaunted, and goes on to say his studies have led him to the theory that prehistoric cave dwellings can be found underneath their neighborhood, even under their own house. No sooner has he said this than the ground caves in under where he's standing and takes part of the floor - and Graeme - with it.
Tim and Bill try to call down to see if he's okay, but there is no response. Bill drops a birdcage down the hole and when, about twelve seconds later, Graeme's voice floats up yelling "OW!" they realize the hole is so deep it takes six seconds for sound to reach Graeme and another six for his voice to reach them. They try to hold a conversation with him and ask if he needs help but this delay makes for much confusion and it takes a long time for them to make any sense of what Graeme is asking. Graeme, meanwhile, is delighted with the prehistoric cave he has found, with its bones and remnants within, and wants to explore it but needs gear and some assistants to do so. Finally, Tim and Bill start throwing him food and clothing down the hole, and agree to come down and help him look around.
Three weeks later, Tim and Bill still haven't dared go down the hole yet. They haven't heard much from Graeme either, so they finally decide to brave it. Bill wears far too much gear - he's a veritable mass of bulging pockets and has a cushion tied to his rear, suction boots on his feet, and a helmet and safety gloves. Time tells him when you rappel you're supposed to dress light but Bill scoffs at the notion. Tim ropes himself to the furniture and some counterweights to serve as Bill's anchor while he climbs down first. Bill disputes this - he thought he was going to be anchor - but Tim has already tied himself up and can't move so Bill has to go down first.
Bill's overstuffed gear gets stuck as he tries to go down the hole. Tim hits him with a mallet to knock him loose. He gives a loud, descending scream to make Tim think he fell, even though he's hanging just inside the mouth of the hole. When Tim finds out he pulled a prank on him he is very angry and tells Bill to get a move on. Bill climbs as far as his rope will reach, the asks Tim for the extension rope. But when Tim tosses it to him he gets all tangled up trying to sort it out. He tugs so hard as he twists and turns, trying to free himself, that the ropes tied around Tim and the other objects in the room start to slip. Tim's pants are pulled off, and when he tries to slack the ropes off to help Bill the rest of him is pulled in the hole, bringing the furniture and counterweights with him. Both he and Bill plunge to the bottom of the pit.
Thanks to Bill's overstuffed pockets, they have a nice soft landing at the bottom of the pit. They find a human skeleton, which they take to be Graeme's, and feel bad for their delay. However, the head moves and speaks to them and they think it is alive, so Tim starts to look for food. Graeme comes out from behind it - he was messing with the head and making it move - and expresses his frustration that it took them so long to get down there. He shows them some of his "Neolithic man" finds and his attempts to reconstruct the bones. Tim and Bill believe that he's done it incorrectly, and Tim says the other bones are not those of early man at all but those of older land mammals, using the size and possible age of certain stalactites nearby to back his claim. Bill in turn disputes Tim's explanation, saying the stalactites are actually T-rex teeth still in their original jaw. They climb into the jaw and discover that is indeed the case, but shortly thereafter the upper jaw drops on the lower one and seals them in.
Tim panics and screams for help. Bill says this will do no good, but then reasons it will do no harm either and joins him. Graeme gets them quieted down and they try to open the mouth. It's too heavy - Tim gets his arm trapped and it's all they can do to just lift it enough to free it. Graeme says they have to find another way out and they start through the body of the T-Rex. They find the organs intact and not decomposed - they enter the stomach but cannot find any openings big enough to get through past that point. They find the remains of many animals, and also the skull and backpack of another explorer. They fear that they will not get out alive. However, they decide to live off Bill's supplies - something Bill does not approve of but he's outvoted in the matter - while they try to figure out a plan to get out.
Six months later they still are stuck, and trying to figure out a way to get out. Bill makes a Home Sweet Home sign but Tim admonishes him for how long it took to do it, saying he should have found something useful to do. Both express boredom and Bill gets the idea that they can pretend to be football announcers. All goes well until Tim starts announcing the referees for Bill's team are kicking all his players out, and Bill starts changing rules to get back at him. They start arguing and Graeme, who has built a computer out of a buffalo skull and is trying to compute a way out of the dinosaur's stomach, tells them both to shut up so he can concentrate. Tim complies but Bill, too bored to stay still, starts singing, and then using another buffalo skull as a pretend motorcycle while he runs around making engine noises. Tim tries to get him to be quiet before Graeme says something, and ends up being the one reprimanded by Graeme for making too much noise.
Bill goes and gets them something to eat - by now supplies have dwindled down to one bean a day, it seems - and then goes to eat a chicken from a private stash he's been hiding from them. Tim catches him and Bill says he refuses to share with either of them. The ensuing argument so escalates that Tim runs out of the stomach and into the spleen to sulk. Graeme takes the opportunity to steal and eat the chicken, and Bill is furious.
Graeme says they should stop arguing, sit down, and work out something together or they'll never get out. Bill yells for Tim to come back, and he does, complaining that there was no need to shout. This gives Graeme an idea - they will use the T-Rex's vocal cords to amplify their own shouts for help so maybe someone one the surface will hear them. While Bill shouts into the vocal cords, Tim and Graeme fiddle with nerves to get the jaws to open a crack so the sound will go out. They send a long message for help this way, then sit back to wait. But it does not succeed - no one hears them, and no one comes.
They make their way back to the stomach. Graeme is confused, wondering why it failed - Tim and Bill are reduced to helpless tears. A loud moan echoes around them - Graeme tells Bill to stop crying and Bill says he didn't make the noise. Tim also denies it. The moan echoes again and Tim worries they may have woken the T-Rex from hibernation, since it hasn't decomposed and isn't really dead. Graeme ridicules this, until the large organ he's leaning against starts drumming and pulsing loudly and knocks him down. Graeme realizes this is the heart and that blood is circulating again. A moment later the stomach begins producing digestive juices. They all panic for a moment, but then Graeme remembers that, having just awakened, the Dinosaur will be yawning and that should give them a window to run out of the mouth. They rush to the mouth - Tim and Graeme get out but Bill takes a wrong turn and ends up in the eye.
Tim tells Graeme they should leave him and collect the insurance. Graeme protests until he hears the insurance is 50,000 pounds. However, Bill finds the way out and joins them and they have to abandon the scheme. They climb back up the pit and into their house. Graeme regrets he couldn't study the T-Rex more but Bill and Tim wave goodbye to it, glad to be away from it at last. However, the T-Rex climbs up the pit and picks up the house, shaking it around and knocking the roof off.