A group of suspects with a pack mentality who kill for the thrill of it are the subjects of a BAU manhunt.
A patient has a severe rash after being exposed to caustic chemicals at work; House must choose between attending a charity event to support Cuddy and staying with his patient; Chase and Masters learn about relationships.
The International Response team heads to Cuba when American bodies are found in different locations throughout the island nation.
Veronica takes her private-investigator exam; Piz interviews a student who wrote a book about his years as an orphan child-soldier in Uganda's rebel army.
Hurley and Sayid find themselves on the run after trouble at the safe house; unknown forces attack survivors left on the island; an old friend has shocking advice for Kate.