Concerned by Chad's unlikely disappearance, Strether goes to the young man's apartment to see if he has returned. Chad is there, having in fact just returned from a trip to London, and he greets Strether warmly. As they talk, it occurs to Strether that he could stay in Paris with Chad indefinitely. This raises a moment of regret in Strether's mind, but he finally tells Chad he has come to bid him goodbye. He also has one more thing to say: "You'll be a brute, you know — you'll be guilty of the last infamy — if you ever forsake her." Chad protests the idea and says he doesn't know what should make Strether feel he is tired of Madame de Vionnet. "Strether didn't quite know, either, and such impressions, for the imaginative mind, were always too fine, too floating, to produce on the spot their warrant." He reminds Chad of all Madame de Vionnet has done for him and says, "I remember you, you know, as you were."
As he walks Strether back to his hotel, Chad suddenly announces a revived interest in the art of advertising, and Strether wonders if he will give up Madame de Vionnet after all. Chad denies the possibility, but Strether's parting remark to Chad is "You're restless."
Strether, in a pure and noble effort, takes the ultimate step on behalf of Madame de Vionnet by appealing to Chad "by all you hold sacred" not to forsake her. Strether senses from Chad's response that he is more concerned with the art of advertising than a sacred obligation to Madame de Vionnet, and he perceives that Chad will prove a source of real disappointment. The reader can see here that it is actually Strether who has been profoundly "changed" by Europe; Chad's alteration has been a superficial one.
Ironically, Strether's original embassy — to effect the return of Chad to America — has succeeded; but, in terms of his new awareness and sensibility, Strether has "failed."