''The Door into Summer'' is a 1957 science fiction novel by Creator/RobertAHeinlein.

Daniel Boone Davis is a genius engineer, an inventor who specializes in pseudo-autonomous robotic home appliances. He made a name for himself inventing house-cleaning robots. When his partner and fiance confront him about optioning his business, he finds himself locked out of his own company, unable to access his work or his designs.

Depressed, a drunken Davis quickly gets tired of running from his troubles and decides to ''really'' run from his troubles; a thirty-year long cryogenic sleep.

Davis wakes up to a changed world. The only people who loved him were his cat and his god-daughter. Some research reveals that she's disappeared and his cat starved to death, abandoned in the countryside near his former partner's home. His carefully managed stocks have disappeared into a Ponzi scheme. He's only got one thing going for him: thanks to his house cleaning robots, he's something of a celebrity.

Davis is soon living a simple, quiet life as an ad man and inventor in the future. There's only one problem: someone has stolen his designs and is publishing them as their own. Some of those designs existed only in his head and were never put on paper. And the name on the patents is D.B. Davis.

What the ''hell'' is going on?

----
!!This novel provides examples of:

* AbsentMindedProfessor: Dr. Twitchell is a brilliant physicist but disillusioned and with a habit of DrowningHisSorrows (he was robbed of fame and Nobel Prize). Daniel manages to talk him into using the top secret time travel machine with flattery.
* AffectionateNickname: Frederica is almost always referred to as "Ricky" and Daniel calls her twice Rikki-Tikki-Tavi.
* AppliedPhlebotinum: The Thorston memory tubes Davis plugs into his robots. Basically, in the absence of personal computers, functional and autonomous robots capable of even a single task (like washing the dishes or vacuuming) were beyond even imagining, so Heinlein came up with the minimum phlebotinum necessary to make them possible: a memory device that would allow the robot to perform a rote task once it's been walked through it.
* BeethovenWasAnAlienSpy: [[spoiler:Either Leonardo da Vinci or Quetzalcoatl or both may have been a time traveler Leonard Vincent.]]
* CompoundInterestTimeTravelGambit: Cold sleep is advertised for that. The danger of managing company going bankrupt is never mentioned to clients.
* GoodPeopleHaveGoodSex: The Suttons, the only trustworthy business partners Daniel found. They share Heinlein's views on nudism.
* GrandfatherParadox: This is boiled down to the time-traveler protagonist waiting just outside of a room where he also is prior to his time-traveling activities, and briefly wondering what would happen if he ran in and slashed his counterpart's throat. He doesn't do it, of course, because that would be stupid and accomplish nothing, but he notes in present tense that he still hasn't figured it out.
* HaveAGayOldTime: Davis almost gets punched out at one point for using a word that was unremarkable in his time, but had become incredibly offensive by the 21st Century. The word in question? Kink.[[note]]Though judging by context when the word is used again later in the book by a different character, it's used in roughly the same way that "goddamn" might be used today, and doesn't seem to refer to sexual perversion at all.[[/note]] Ironically, several pages earlier he used the word "fagged" as if it was still a word people use to mean "fatigued" in the year 2000, when it was already a dirty word by 1970 in real life.
* HumanPopsicle: Starts out being based around this trope. [[spoiler:Until the protagonist sees a bunch of things in the future he woke up in that can only be explained by his eventually gaining access to conventional TimeTravel.]]
* IWantMyJetPack: Commercially available cryonic suspended animation and robots in 1970.
* TheJailBaitWait / WifeHusbandry: [[spoiler:Ricky waits till she's 21 and goes into cold sleep ordering to wake her when Daniel finds her. He is in his early 30s when they marry.]]
* KindheartedCatLover: Daniel, although he owns only Pete. Pete can be used as an indicator if a person can be trusted. This was the first sign to beware of Belle.
* MeaningfulName: Belle S. Darkin. Belle = beautiful; Darkin = darkly figure who should not have been trusted.
* PetsHomageName: Dan, brilliant engineer on the cutting edge of domestic robotics, named his cat Petronius the Arbiter.
* PrecociousCrush: Eleven-year-old Ricky loves thirty-year-old Daniel. When she was about six, they used to play a game (which she took seriously) that she's his girlfriend. When Danny starts dating Belle, Ricky instantly dislikes her and gets jealous.
* SealedArmyInACan: Discussed, but subverted. [[spoiler:Time machine cannot be used to transport troops, because nobody can predict what goes to the past and what goes to the future.]]
* StableTimeLoop: [[spoiler: The protagonist travels into the future and sees machines he's almost sure he invented. So on that hunch, he finds a time-machine that can send him back. He makes some arrangements, returns to the future by [[HumanPopsicle cold sleep]] and lives happily ever after knowing the people who tried to ruin his life got their just deserts.]]
* SuddenlySignificantCity: Denver became the capital of the USA after [[WorldWarIII the Six Weeks War]] devastated the Northeast.
* TeleFrag: Discussed. If a time traveler ends inside a solid object, for example a tree, it may result in an explosion. Daniel takes this risk and fortunately doesn't hit any trees. The exact words: "Ought to make quite an explosion, about like a [[http://en.wikipedia.org/wiki/Cobalt_bomb cobalt bomb]], huh?" It's unclear if this is [[UnreliableNarrator Chuck's mistake]], or [[ArtisticLicenseNuclearPhysics the author's]].
* TitleDrop: In the prologue, Davis describes how, in the winter, his very outdoorsy cat is thoroughly discontented. When there's snow on the ground, he refuses to go outside, but also refuses to accept that it's snow ''everywhere'' and voices his complaint that Davis can't control the weather. So the two follow a daily routine where Davis and the cat patiently explore the home, looking for The Door Into Summer.
* TwentyMinutesIntoTheFuture: When the book was written, the 70s were twenty minutes away, but then Heinlein jumped it another twenty minutes into the early 21st century.
* WiseBeyondHerYears: Ricky is eleven, but extremely reasonable. For instance, Danny notes she takes care of her step-father's household.
* WorthlessYellowRocks: Synthetic gold is cheap in 2001. Daniel can buy 10 kilograms with his paycheck.
* WriteBackToTheFuture
* YouAlreadyChangedThePast: Various instances of HumanPopsicle, but more importantly a weird time machine that has an equal chance of throwing the subject forward or backward. The protagonist uses it knowing he HAS to be sent backwards. Bonus points to a throwaway gag that suggests that [[BeethovenWasAnAlienSpy Leonardo da Vinci is (and always has been) an accidental time-traveler]].
* {{Zeerust}}: A bit worse than most. His assumptions about the 70s were bad enough, but when you get to the 21st century, it starts to look a bit silly. As per usual, Heinlein never really anticipated computing and his personal robots (we ''do'' have roombas now) run on gears, electrics (''not'' electronics) and "memory tubes". The groundbreaking "Drafting Dan" is essentially a typewriter for engineering designs. Still better than a drawing board in a world without CAD software.