In a nutshell, Kugelmass (a stereotypical Jewish name), is an unsatisfied Humanities professor. He decides to have an affair since his second marriage is not a happy one. A magician has a Magic Box that Kugelmass can crawl into with a favorite book, and he'll end up in the setting of that book. He chooses Madame Bovary. He and Madame Bovary get along well and start an affair--he even brings her back to his time. Ironically, everything they do ends up within the text of Madame Bovary. Kugelmass' fellow professors recognize him in the descriptions and pages of the book. Curiouser and curiouser.
Emma Bovary proves to be too much to handle--in short, he can't keep her happy either, so he sends her back into the book. He decides to try traveling by box to another setting, but the box has a technical difficulty and Kugelmass ends up in a Spanish textbook being chased by the verb "tener"--"to have". Basically, the verb (large and hairy...UGH) wants to acquire him as he wanted to acquire Madame Bovary thinking that an affair would change his mood. No more free will for Kugelmass--he is at the mercy of "tener".
I have included some links for you which may make some of the irony and comparisons easier to understand. It wouldn't hurt to look up the Jewish terminology that you don't understand in order to see the point in the story more clearly, either. Good Luck!