Mrs. Sparsit, after losing Louisa at the train station, goes to London to find Bounderby. She tells him everything she knows (or thinks she knows). He hustles her onto a train, and together they go back to Coketown and barge into Gradgrind's house.
Bounderby starts yelling about Louisa and Harthouse, when Gradgrind interrupts him and says that Louisa is actually not with Harthouse but is upstairs. Bounderby then starts yelling instead at Mrs. Sparsit, then quickly shoves her into a carriage to be taken home.
Still pretty angry, Bounderby comes back inside and has a long argument with Gradgrind about Louisa, the state of their marriage, and Louisa's apparent desire to separate from him for a while.
Gradgrind is a lot more human than he has appeared before – and this makes Bounderby even more of a heartless jerk than he already is.
Gradgrind says that it is probably best that Louisa stay with him for a little while and deal with all her mental and emotional issues. He also points out that the marriage is a pretty bad one. Bounderby goes nuts and gets defensive, telling Gradgrind that the real problem is that Louisa doesn't appreciate Bounderby nearly enough.
Bounderby makes an ultimatum – either Louisa comes back to his house by noon the next day, or else he will consider the marriage null and void.
The next day, Louisa does not return.
Five minutes after twelve noon, Bounderby tells his servants to pack up all his stuff, sell the country house, and forget he was ever married. (Legally, of course, they are still married to each other. It's probably best to think of them as what we now call legally separated.)