I read the first three books of the Hall Family Chronicles when I was a girl. I was lucky enough to find hardcover copies of all three at the same library used book sale in 1982. I decided to reread them all again over this past weekend.
Prince Krishna has built a lovely summerhouse for Eleanor and Eddy Hall. This is no ordinary summerhouse. There's a magical adventure built into each archway. You enter the adventure by jumping off the swing. Once the magic starts, there's only so many swings until the summerhouse disappears forever. The bugs haven't been worked out of the last adventure before Prince Krishna has to fly back to his native country to avert war. Prince Krishna takes precautions by boarding up the archway, painting "KEEP OUT" on the boards, getting Eddy and Eleanor to promise they won't go through that archway, and not putting a swing in the summerhouse.
Prince Krishna's precautions are in vain. Eddy hangs a swing in the summerhouse. Of course Eddy wouldn't break his promise, but his obnoxious friend, Oliver, didn't promise anything. Oliver sees the "keep out" notice as a challenge. He swings until he breaks the boards with his feet, then sails off the swing -- and vanishes! What's poor Eddy going to do? Oliver's parents are very worried, but he can't tell them that Oliver jumped off the swing and disappeared into thin air. They'd think he was nuts. Eddy can't go through the SAME archway because of his promise, but that doesn't mean he can't go through the OTHER archways.
Meanwhile, Eleanor is very suspicious of Uncle Fred's new student, Mrs. Dorian, who has moved next door along with her little girl, Georgie, who isn't even in school yet. Eleanor is sure that Mrs. Dorian is a witch and has bewitched Uncle Freddy, because her uncle acts so silly around her. If you're older than Eleanor, you can probably figure out why.
Eleanor and Eddy don't get along as well as they did in THE DIAMOND IN THE WINDOW. Eddy has become a normal, messy boy who loves junk. He makes Eleanor so mad because she's become a prissy prig. Eleanor can hardly wait until she's old enough to wear pretty dresses and go to dances. She hates the fact that her uncle's school isn't making them rich because Uncle Freddy cares more about teaching than making money. Will the summerhouse show Eleanor the error of her ways?
My favorite archway adventure was "Make New Worlds". Sure, the man-castle and the big clipper ship were neat and the giant cash-register can teach you how much you're worth, but what's that compared to having the worlds you create come alive? (If you don't like the world of Princess Cinderellanor, wait until it's invaded by Eddy and Georgie's worlds!)
Time goes by. There's a tick-tock every time the swing is swung. All the adventures before the dangerous one have been tried, but Oliver is still missing. Eddy boarded up the archway after his friend disappeared, but Georgie is tempted. Georgie wants so much to be able to read and add two and two. Georgie is sure that she'll get what she wants if SHE breaks the boards and goes through. Will she? Of course she will. Who's going to rescue her (not to mention Oliver)? Eddy and Eleanor don't know it, but time is running out....