Carroll Baker plays a dual role as translator Julie Harrison and her twin sister Mary. The serpentine plot begins as Julie tells her lawyer Dave Barton that Mary's life is being threatened in London while Julie herself is being stalked by a mysterious stranger in Amsterdam.
| Actor | Role |
|---|---|
 Carroll BakerJulie Harrison / Mary Harrison | Julie Harrison / Mary Harrison |
 George HiltonTony Shane | Tony Shane |
 Stephen BoydDave Barton | Dave Barton |
 Lucretia LoveMargaret | Margaret |
 Luciano PigozziSteve Hunter | Steve Hunter |
 Daniele VargasJames Marlowe | James Marlowe |
 Ivano StaccioliHank | Hank |
 Franco ResselInspector Rinker | Inspector Rinker |
 Carla ManciniNot listed | Not listed |
 Gianni PuloneNot listed | Not listed |
 Roberto MessinaNot listed | Not listed |
| MMaria Carla RicottiNot listed | Not listed |