We departed Bremerton Washington for Sun Valley Idaho last week during the fires. Skies were reported clear above the smoke with tops unknown. We figured 15K would easily clear the smoke based on our past experiences with summer fires. We left Bremerton IMC and climbed through the smoke before breaking out at around 10k. Breathing our cannulas, we were feeling pretty good about ourselves and choice to leave as we figured clear skies for the next 3 hours. Unfortunately, our overconfidence was quickly met with a new reality - the smoke over the cascades quickly engulfed our RV-10 to the point were it was beginning to feel a little unsafe. The cannulas weren't enough to keep the smoke from going into our bodies and irritating our throats. I tried asking for ATC for the tops as we still had a few thousand feet left in our ceiling, but was told another plane just reported smoke up to 25k. With some quick thinking we rescued our masks from the bags - never used - and instantly felt better after pumping up the oxygen flow. The next hour and a half was spent in thick smoke until we broke out again around Boise. We were prepared and fortunate to have brought a full tank of oxygen, and have our masks on hand, but I don't think we would have made that flight again. The smoke was more intense and thick than we'd ever experienced at 15k - here is a shot out the window to show it was seriously thick even this high.