A young man swimming off the Massachusetts shore collides with tropical jellyfish whose stings trigger violent flashbacks to his childhood. A docile elderly woman slashes the throat of a CVS clerk with a pair of $3.39 scissors. A pharmacist notices a bizarre trend among Alzheimer’s patients at a nursing home: Their memories are improving. Gary Braver brews a dandy thriller from a grisly 30-year-old murder and the shenanigans of a sleazy drug company trying to speed approval of a risky Alzheimer’s cure. And though it takes a while to feel its effect, Flashback offers a potent hit of suspense.