It's been about a year since Napoleon and Illya have seen Gervaise - and Illya killed Harold - and now Gervaise has escaped from prison and is out for revenge. That revenge comes in the form of having Illya's mind programmed so that he allows her thugs to kidnap Napoleon as he's leaving Illya's apartment one evening.
When Napoleon doesn't show up the next day for a meeting, Illya's memory of the programmed events come back to him. Deemed unsafe due to the programming, he's kept in the infirmary at UNCLE.
Meanwhile, Napoleon's held hostage by Gervaise and subjected to repeated torture - and photographed, with the pictures being sent to Illya in order to make him suffer, as well. Gervaise, it seems, is under the impression that Napoleon and Illya are lovers. This surprises them both, but a separate set of photographs makes clear to each of them just why Gervaise had that impression.
The ‘Til Death Do I Trust Affair, by RAC