A mysterious killer is attacking women associated with a blackmail conspiracy. The deranged murderer, wearing surgical gloves, kills his victims by paralyzing them with a needle and then slicing open their bellies with a knife (in the same way tarantulas are killed by the black wasp or spider wasp). The victims are conscious and can feel the pain, but they are unable to move, resist or scream. One victim (Barbara Bouchet) is wearing a yellow nightie when she is killed, an obvious nod to the ''giallo'' covers of the lurid Italian murder mysteries on which the films were based. It is up to the reluctant Inspector Tellini (Giancarlo Giannini) to find out who the killer is, before he or his girlfriend become the murderer's next target.