Formerly appearing on a recurring basis as forensic psychologist Dr. George Huang, B.D. Wong graduates to full series regular in "Chameleon," the opening episode of Law & Order: Special Victims Unit's fourth season. Among the year's best episodes are "Vulnerable," guest-starring former musical comedy ingenue Jane Powell as an elderly Alzheimer's victim who is misused by both a sexual predator and her own caregiver; "Disappearing Acts," in which the SVU team runs smack against the stone wall of the Witness Protection Program while tracking down a brutal rapist; "Waste," a compelling exploration of the dark side of stem-cell research; and "Risk," wherein the team must expose the rotten apples in their own police ranks to find out who caused the cocaine-related death of a baby. As in previous seasons, many of the plotlines are driven by the unique personalities of the principal characters. In "Mercy," for example, Assistant DA Alex Cabot (Stephanie March) finds herself sympathetic to a mother who euthanized her desperately ill daughter, and in "Pandora," Det. Elliot Stabler (Christopher Meloni) travels all the way to Prague to crack a difficult kidnapping case. Other episodes emulate the Law & Order "ripped from today's headlines" formula. Case in point: "Appearances," which is clearly inspired by the still-unsolved murder of JonBenet Ramsey. Season four ends with "Soulless," chronicling the frustrations of using the juvenile record of a suspect to arrest him for sex crimes committed as an adult.