Yes, Virginia - they did have slasher movies before Halloween and Friday the 13th. Not very good ones, true, but the core elements can certainly be found here: a lunatic, seeking vengeance, taking out those responsible, one body part at a time. Here, prime suspect is Ted, who lost an arm during a caving expedition. Actually, after two weeks trapped by a rock-fall, his compatriots cut it off as food. They were rescued before needing to chow down, but claimed Ted was merely delirious rather than 'fessing up. If I was Ted, I'd be pissed too. Anyway, five years later, one of the wannabe-cannibals gets a severed arm in the mail, and soon, his friends are losing limbs like autumn leaves. Can they find Ted before any more 'arm is done? [Sorry!]
The pacing on this is definitely sluggish, and the viewer will be several steps ahead, looking over their shoulder, waiting for the script to catch up. But it's nice to see an early example of "The phone call came from your other line!" If Marvin Kaplan as Mad Man Herman sounds familiar, it's because he voiced Choo Choo in Top Cat, and Herman actually has a spark of life, more than can be said for the rest of the cast. There's one impressive example of arms reduction, but otherwise this is too low on the evolutionary chain to be of much interest nowadays. Keep banging the rocks together, guys.