Children of the Lens by E.E. Smith

When Kandron called his minion in that small and nameless base to learn whether or not he had succeeded in trapping the Palainian Lensman, Nadreck's relay station functioned so perfectly, and Nadreck was so completely in charge of his captive's mind, that the caller could feel nothing out of the ordinary. Ultra-suspicious though Kandron was, there was nothing whatever to indicate that anything had changed at, or pertaining to, that base since he had last called its commander. That individual's subconscious mind reacted properly to the key stimulus. The conscious mind took over, remembered, and answered properly a series of trick questions.

These things occurred because the Base Commander was still alive. His ego, the pattern and matrix of his personality, was still in existence and had not been changed. What Kandron did not and could not suspect was that that ego was no longer in control of the commander's mind, brain, or body; that it was utterly unable of its own volition, either to think any iota of independent thought or to stimulate any single physical cell. The Onlonian's ego was present -- just barely present -- but that was all. It was Nadreck who, using that ego as a guide and, in a sense, as a helplessly impotent transformer, received the call. Nadreck made those exactly correct replies. Nadreck was now ready to render a detailed and fully documented -- and completely mendacious -- report upon his own destruction!