So the book starts right after the events of book 2.
The couple had a marriage of convenience and safety for the heroine as it offered her protection but it was also an open marriage, both had been with others.
After the heroine's declaration and the time they had spent together recently the heroine felt things had changed so she was really, understandably hurt that he still went on to deliberately hurt her by bringing two women back to the house and even expecting them to eat breakfast together.
The Hero is basically running scared of anything 'real' between them.

They are separated at this point, or at least at the start she is leaving him.
The wife sees another man but the husband is celibate in their time apart.
Brave of the author to go against the norm there.
Readers tend to forgive Heros actions but not so much the heroine.
From the start of the book the Hero realises he went too far and is all about making amends.
At first his motives or feelings arent clear but he really tries to make it up to her and the heroine stands firm that she won't take him back.
It takes time.
No intimate scenes with anyone other than the Hero and heroine.
HEA.