Louise Haloran (Luana Anders) is a pretty, greedy young lady who, when her husband John (Peter Read) dies of a heart attack, does not hesitate in hiding his body and pretending he went away on a business trip in order not to lose the inheritance.
So she travels to Ireland to meet her husband's mother, Lady Haloran (Ethne Dunn), and his two brothers, Richard (William Campbell) and Billy (Bart Patton). She finds a strange family, still haunted by the tragic death of the youngest child, sister Kathleen, when she was just a little girl.
And someone has decided to start axing people to death...
This film contains the examples of:
- Accidental Murder: How Billy killed Kathleen.
- An Axe to Grind: As Louise found out firsthand.
- Axe-Crazy: Billy, and their weapon of choice was actually an axe.
- Alas, Poor Villain: Billy's death is probably the saddest scene in the movie.
- Asshole Victim: Louise had it coming.
- Cymbal-Banging Monkey: How to make the monkey even creepier? Have him wielding an axe instead of banging cymbals.
- Decoy Protagonist: Louise plays the Janet Leigh part in this one, getting brutally murdered less than halfway through the film.
- Doctor Jerk: Caleb. He abuses the help and snarks off to Richard and Kane constantly, for no readily apparently reason.
- Establishing Character Moment: Louise throwing John's dead body into the lake.
- Foreshadowing: The scene where Billy idly toys with the hatchet, as well as the Cymbal-Banging Monkey chopping away with an axe.
- Gaslighting: Louise starts trying to convince Lady Haloran that her daughter's ghost is communicating to them, presumably in order to get Lady Haloran to change her will.
- Gold Digger: Louise, who wants that family money, and is willing to chuck her husband's body in the lake and pretend he's still alive in order to get it.
- Hazardous Water: So, um, why doesn't Louise swim away after the axe comes out?
- Jerkass: Louise, who is greedy and likes to manipulate old ladies.
- Karmic Death: Louise pretty much deserved that ax.
- Non-Indicative Name: Dementia 13. The 13 is completely random, and whatever mental illnesses we're dealing with here, dementia isn't one of them.
- Supposedly, the title was just supposed to be Dementia and the "13" was added when the creators discovered the existence of another recent film by that name. Depending on who you ask, they went with the number 13 because that was the age at which the killer committed his first murder, or because Roger Corman just thought it was a "scary-sounding number."
- The Scourge of God: The two victims who actually die in the movie were actually doing wrong, and the one who was worse got the worst death.
- Whole Plot Reference: Pretty much a blatant ripoff of Psycho, but directed with a visual flair indicating Coppola as an up-and-comer. Corman in fact specifically asked Coppola to make a Psycho knockoff.
- Woobie, Destroyer of Worlds: Billy.