Die, Monster, Die! has a problem. It can't decide whether it wants to be a 1950's cautionary tale about the dangers of radiation or a 1960's "there's something dark and Satanic lurking around the house" story. It's a bit confusing to the viewer. The original source material is a story from H.P. Lovecraft that I have not read...yet. If you have, it would be interesting to see how the two differ. Something definitely got lost along the way.
This is a shame because the performances in this film are quite good in spite of the ambiguities in the script. The story begins when a young man (Stephen) visits his fiance's estate (Susan). He discovers there a less than hospitable father (Dr. Whitley) who is wheelchair bound and wants Stephen to leave the estate ASAP. Boris Karloff [Frankenstein, The Mummy] plays Dr. Whitley with great depth and passion. Karloff always has a certain spark in his performances that is enjoyable to watch no matter what decade we find the actor perfecting his craft. Nick Adams [Frankenstein Conquers the World, Voyage to the Bottom of the Sea] and Suzan Farmer [Dracula: Prince of Darkness] are also quite good as the young lovers.
Furthermore the cinematography is quite beautiful and reminds me of the best of the Hammer Horror pictures. The colors are vivid and the sets are impeccably constructed. The film is directed well, too. There's just the problem of the script that I think needed a major rewrite. Oh well, if you're a Karloff fan, you'll want to see this one. If you're not there are definitely better films out there from the 1960's to watch.
For more info check out the film's entry in IMDB.