The title refers to the purported weight we immediately lose when we die. Director Alejandro González Iñárritu (Amores perros) explores the theory that this effect comes from one’s soul departing to the other side. He entangles the lives of three strangers after an accident takes the life of a man and his two children. Paul (Sean Penn) receives a heart transplant and must face his own mortality. Cristina (Naomi Watts) is crushed by personal loss and seeks revenge. Jack is a reformed ex-con enraptured by newfound religious zeal (Benicio Del Toro). Iñárritu fractures time into an intriguing nonlinear rumination on redemption and on the dark side that lies within us all.