Just as Dan finally figures out the full extent of the swindle, Belle and Miles drug him and put him into cryogenic suspension for 30 years, forcing him to leave behind the only three things he cares about: his business; his cat, Petronius the Arbiter (aka Pete); and his 11-year-old adopted niece, Fredericka (aka Ricky).

When he awakens in 2000, Dan faces the daunting task of trying to survive in a strange world where his engineering skills are decades out of date. After much time and effort, he finally manages to land a job at his old company, befriending Chuck, their most competent engineer. He cannot locate Ricky because he does not know what last name she adopted, but his search uncovers many anomalies indicating that he did things back in 1970 that he cannot remember. When Chuck tells him about a scientist, Dr. Twitchell, who has invented a time machine that has been classified top secret by the military, Dan realizes what he must do—indeed, must already have done—to set things right.

He visits Dr. Twitchell and tricks him into sending him back to 1970, before he was swindled. Landing in the middle of a nudist camp, he meets and befriends John and Jenny Sutton, who become his new and trustworthy partners. Dan then spends the coming months assuring that Belle and Miles do not benefit from their swindle, and that when he returns via cold sleep to 2000 for the second time, his business, his cat and his new fiancée (Ricky 10 years older) will be there with him.

The ultimate ode to optimism

The Door Into Summer is Robert Heinlein's most upbeat and enjoyable adult science-fiction novel. The title represents one of the most memorable metaphors in all of science fiction. Whenever there is inclement weather, Dan's cat demands that each door of the house be opened to try to find a door into summer. The clear message of the novel is that you can make things better if you are resilient, smart and resourceful, and this upbeat theme makes the book as readable now as it was 50 years ago—and assures its continued place as one of the most popular science-fiction novels of all time.

The theme of undeterred optimism and tenacity runs throughout the story, as Dan overcomes one impossible situation after another through cleverness, persistence and never-say-die optimism, always finding his own door into summer. Your most trusted allies swindle you? No problem, find new friends you can trust. Lose your business, cat and best friend? No problem, find out how to travel back in time and make things right. Your future fiancée is only 11 years old? Dan finds a way to fix even that problem. Indeed, Heinlein's whole future society demonstrates this resilience—the novel is set after a nuclear war has destroyed Washington, D.C., and New York City, but the nation's capital has simply moved to Denver, and by 1970 all is well.

Dan's relationship with Ricky is one of the more talked-about issues with The Door Into Summer, along with the arguably simplistic characterization and some questionable technological and social extrapolations for the future years of 1970 and 2000. Starting with the last issue, I continue to be impressed with the general quality of Heinlein's technology extrapolations. His only real miss is projecting robots instead of computers being the driver for technological change, and, along with all SF authors prior to NASA, grossly underestimating the complexity of our future technology, which requires the work of vast teams of engineers and scientists, not a single talented inventor. His social extrapolations are not particularly astute, but that is true of most SF prior to the late 1960s.

On the issue of sparse characterization, I believe it is more than adequate—Dan is the archetypical competent Heinlein engineer familiar to all SF readers, and we know what he would do or say in a wide variety of situations well beyond those in this novel. But this rational-man archetype does not have emotions per se—Heinlein portrays emotions primarily as a form of temporary insanity that leads to critical mistakes. This is why some early critics expressed concern about weak characterization, and also why Heinlein did not see Dan's desire for Ricky as pedophilic. Thirty-year-old Dan was not in love with 11-year-old Ricky; he merely made a rational judgment that she would grow up to become his perfect mate.

The Door Into Summer is the ultimate feel-good novel, and perhaps should be prescribed to anyone in need of a boost in morale.