“The Flash”: The CW, 8 p.m. Tuesday

Season premiere. When an experiment to save Barry’s (Grant Gustin) speed backfires, Nash Wells (Tom Cavanagh) searches for a way to save the Flash and comes up with a dangerous plan. Meanwhile, Iris (Candice Patton) makes a startling realization inside the Mirrorverse and Cecile (Danielle Nicolet) faces off with Rosa Dillon (guest star Ashley Rickards).