Although evidently a recent recruit, Chuck Armstrong is assigned to protect Agent Roberts when the latter's life is threatened. However, it transpires that Chuck is in fact "Chucko", a killer ape transformed through "a series of injections" administered by renegade scientist Dr. Mathew Rath so that he looks human. Rath is being paid handsomely by KAOS to kill CONTROL agents and Roberts becomes his seventh victim. All received a banana in the mail the morning of their deaths - Chucko's incentive and reward for the killings.
When Maxwell Smart receives a banana, Armstrong is assigned as his bodyguard and taken to the Smarts' apartment to spend the night. While opening a bottle of wine in his human guise, he cuts his finger and is deeply moved by 99's kindness in applying a bandage. In a reaction reminiscent of Androcles and the Lion, after Dr. Rath activates him as "Chucko" he is unable to harm her and instead carries her with him up to the roof of the apartment building.
Meanwhile, the Chief has learned the truth about Armstrong and telephones Max from Rath's laboratory. Thus forewarned, Max follows Armstrong to the roof. After failing to overcome him physically, Max offers him the banana which, despite specific instructions from Rath that he is to have it only after the murder, he eats seated on the ledge - all the while totally ignoring more blows from Max. Then, standing, Armstrong turns to attack Max only to slip on the discarded banana peel and fall to his death.
The only mystery remaining is why the Chief was sufficiently impressed by what he called Armstrong's "excellent work" to assign him as Max's bodyguard in the first place after he had failed to protect Roberts in the same circumstances.