A Delta Quadrant species who developed an experimental coaxial warp drive capable of folding space, and whose test pilot Steth had his body stolen by a shape-stealing fugitive.
The Benthans are a technologically ambitious humanoid species seen in the Voyager episode Vis à Vis. Their most notable achievement is the coaxial warp drive, which folds space so that a ship can cross great distances almost at once, though the engine is dangerously unstable.
Voyager rescues a prototype coaxial ship flown by a man calling himself Steth, and Tom Paris helps to repair it. The pilot is really an alien who survives by exchanging bodies with others. He takes Paris's form and leaves Paris in Steth's.
Benthan patrol ships pursuing the stolen prototype catch up with Paris, and with them comes the real Steth, a Benthan test pilot trapped in a woman's body. Together they return to Voyager, where the impostor has by then moved on to Janeway, and everyone is restored.