Robert A. Heinlein · 1951 · thriller

Heinlein's 1951 invasion novel, set in a 2007 imagined from the height of the Cold War. Slug-like parasites land in Iowa and start riding people's backs, controlling them completely - and the only reliable way to tell who is host and who is not turns out to be mandatory nationwide nudity.

1. 1
   Grinnell

   Summer 2007 as imagined from 1951, and an unidentified disc about a hundred feet across lands in an Iowa field - reported, then announced as a teenage hoax, and then silence from the six agents sent to look at it.

2. 2
   Des Moines

   The Old Man goes in with Sam and Mary and finds the town already taken - a slug on the back of a television executive, which is how the news of an alien invasion gets censored at source.

3. 3
   Washington

   They cannot get the President to declare an emergency, and a captured slug escapes and takes Sam - who then uses the Constitution Club, full of the city's political class, to find the parasites new hosts.

4. 4
   Des Moines

   Back into the zone for evidence, and the policy that follows: Schedule Bare Back, mandatory nudity nationwide, because a slug can only be seen on a bare back - which is Heinlein taking a premise entirely seriously.

5. 5
   Kansas City

   Sam walking a warm street and counting the people dressed for the weather, and finding too few - the moment the reader understands how far it has spread and how simple the test for it has become.

6. 6
   Washington

   Operation Vengeance, and the answer: the slugs come from Titan, and the weapon is a disease. The book ends with a fleet leaving to take the fight there, which is not how invasion novels usually finish.