Solo and Illya try to protect Professor Lillian Stemmler from THRUSH after she invents a drug called Plus X, which heightens all the human senses. Unbeknownst to them, she is a THRUSH agent herself who at first cooperates willingly, then has second thoughts. But THRUSH agent Rollo kidnaps her daughter Leslie and forces her to cooperate. The drug is given to three THRUSH agents who will attack a U.S. government plutonium plant using their superior senses, while at the same time the guards will be given a dose of Minus X, which dulls the senses.
