High about the Arctic Circle (in a sunny tree-filled forest), a group of copper miners collect core samples from below the permafrost. The drill bit comes back up with thick leathery flesh caught in the threads... and fresh blood. Scientists from the Danmarks Aquarium are sent to inspect and dig up the fossil--or whatever is down there. They find part of a giant prehistoric reptile's tail. Keeping the sample frozen, they ship it back to the aquarium for study. The freezer door is left open one night, and it's discovered that the thawing tail is still alive and regenerating. Later, two UN representatives join them: a stern US brigadier general and a beautiful female scientist from UNESCO. Just in time, too, as the combination of increased nutrient flow and an electrical storm cause the regeneration to rapidly accelerate...
The real-life drama behind the making of the film is perhaps more entertaining than the movie itself. American producer/director Sid Pink developed the idea for the story, and co-wrote the screenplay with novelist and screenwriter Ib Melchior (who was born in Copenhagen). Pink set up production for the film in and around Copenhagen. The original version was filmed in the Danish language by Danish director Poul Bang and released there in 1961.
Pink directed the English version for American-International Pictures--twice. The first English version was filmed with many of the same actors, none of whom spoke English. They recited their lines in English phonetically. The execs at AIP, particularly Sam Arkoff, hated it. They had the film re-cut, with American voice actors dubbing the lines in English. They still had to match the stilted mouth movements of the Danish actors, however.
Several scenes and parts of scenes were cut for the AIP release, including the infamous Reptilicus flying scene (or this link). It's set at night so the crude nature of the effects work isn't as obvious. Compared to the rest of the creature effects in the movie, the flying scene really isn't that bad. The model for Reptilicus itself appears to be a marionette puppet.
One aspect of the film that was kept is the inclusion of popular Danish comedian Dirch Passer. He plays the night watchman hired on to keep an eye on the frozen Reptilicus tail in the aquarium. His scenes are played for laughs, of course. The officer he talks to at the police station was one of his frequent comedy collaborators.
For the AIP release, Reptilicus' green acid slime spit was added. This takes the form of hand-drawn animation added into scenes where the monster attacks crowds at the beach or on the streets of Copenhagen. Apparently many, many locals were used for the crowd scenes, and members of the Danish military and their equipment were provided for the films use. These scenes and the stock footage are carefully edited together. It's really not bad at all for a lower-budget production. And, as previously noted, it was a source of pride for the Danish people.
Before the monster attacks, the American general accompanies his Danish counterpart and the UNESCO representative for a drive and a night on the town. This part seems almost as if someone slipped a tour package film into this monster movie. For someone who seemed to hate his new assignment at first, the brigadier general now appears to know all about the history, architecture and public sculpture of Copenhagen.
the 10-minute version. For more details, I recommend the reviews and background at Jabootu's Bad Movie Dimension or The Monster Shack. It's goofy fun, and really inspires the viewer to provide an ongoing MST3K-style commentary.
Update: Thanks to io9 for linking to this review!
Update 2: Shout Factory released a four-movie Sci-Fi Classics disc in September 2013 including Reptilicus, presumably using the same print as the MGM disc.
Update 3: Scream Factory, a sub-label of Shout Factory, will release a Blu-ray double-feature of Tentacles (1977, also previously released in the Midnite Movies series) and Reptilicus on June 16th, 2015.
Update 4: Reptilicus is the experiment for the first episode of Mystery Science Theater 3000's 11th season, now streaming on Netflix (listed as Season One).