[4] Gudenberg, 1890: Professor John Mayer has invented a ray gun which runs on nuclear power.

The aliens decide to come to Earth to destroy the weapon... Karloff's advanced age was noticeable on the film; regarding his health status, Michael J. Weldon in The Encyclopedia of Film noted, "The ill, 81-year-old horror star is always shown sitting down or leaning against a support of some kind.

"[5] AllMovie's synopsis of the film states, "The filmmakers barely had enough talent to adhere to the simplest of storylines, much less this hodgepodge of cut-rate H. G. Wells posturing and sleazy exploitation.

"[2] James O'Neil in Terror on Tape called The Incredible Invasion "one of the four awful low-budget U.S.-Mexican co-productions the great actor [Boris Karloff] filmed a few months prior to his death in 1969."

Michael R. Pitts in Columbia Pictures Horror, Science Fiction and Fantasy Films, 1928–1982 said that "While Karloff is quite good as the scientist and Christa Linder is strikingly beautiful as his niece, the overall production is labored", and that "[w]hile not as bad as Fear Chamber or House of Evil, The Incredible Invasion must rank as one of Karloff's worst cinema outings.