Four teen-agers find a time-ship on a lonely stretch of shore. It has just brought two travelers-in-time from the future. When they leave the ship, Clive, Mark, Carol and Jill climb in. The ship starts off and they are carried on through the centuries at breathless speed until it comes to rest in A.D. 2260 and they step out into an unrecognizable England.

First the sea was pushed back to make room for the ever-increasing population, then cultivation was carried out in layers, with fields rising in tiers like giants' skyscrapers. The people are governed by an enormous electronic brain. The inhabitants of this extraordinary world plan to keep the children forever as curiosities from the past. But the four escape and find their way to the time-ship. They have learned how to work it, but not properly, and they overshoot their own era and return to 1960 BC, coming to rest in the middle of a swamp inhabited by men of the Stone Age.

These primitives are almost the last of their kind, for already Bronze Age warriors with superior weapons are overrunning them, and the children watch with awe a battle between the two opposing forces.