Property Damage from faulty plumber hired by contractor

[Freiter]

We are in California. We had our 2nd floor bathrooms remodeled a few months ago. Last week our new bathtub leaked into our first floor dining room. The water cracked our plaster ceiling, paint , not to mention whatever water damage was done inside the ceiling of our diningroom which is directly under the bathroom. The contractor is blaming the plumber. The plumber is blaming me. He says I probably broke the toggle in the drain by standing on it when it was in the "up" position.; I guess which loosened something causing the drain to leak. I want to know what rights we have. Who is at fault and will repair all the plumbing damage and water, plaster and paint damage. Do we need to open the ceiling to inspect for damage? Do we need to hire an inspector from the city? The contractor also never did a final inspection of the house after the remodel.