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Chaos Theory is the first episode of the ninth season of ER, and the one hundred eighty-first episode overall.
Guest Stars: Paul Freeman (Dr. Charles Corday), Paul Hipp (Craig Turner), Laura Ceron (Chuny), Deezer D. (Malik), Troy Evans (Frank), Lily Mariye (Lily), Harrison Page (Stan), Monte Russell (Zadro), Diane Delano (Stella Willis), Dee Freeman (Dr. Lutz), Christopher Grove (Dr. Marty Kline), Jillie Mack, Michael Muhney (Colin Prentiss), Jim Piddock (Grumpy British Doc), Heidi Swedberg (Robin Turner), B.J. Ward, Judy Parfitt (Isabelle Corday), John Aylward (Dr. Donald Anspaugh), David Hayward, Demetrius Navarro (Morales), Perry Anzilotti (Perry), Philippe Bergeron (European), Allen Bloomfield (Marty), David Allen Brooks (Gunn), James Weber Brown (Passenger), Kaline Carr (Dr. Stephanie Rioux), Crispin Bonham Carter (Passenger), Phillip C. Curry (IDC Doctor), Julanne Chidi Hill (Mother), Claire Jacobs (Mrs. Daniels), Shirley Jordan (IDC Analyst), Miranda Kwok (Kristen Shoop), Dinah Lenney (Shirley), Ruth Manning (Mrs. Newberry), Charles Martinet (Eddy), John C. McDonnell (Nick), Alpheus Merchant (Elderly Man), Kris Murphy (Flight Nurse), Horace Knight (Boat Assistant), Deji Olasimbo (Tye), Eric Payne (Police Officer), Tequan Richmond (Boy), Toby Rolt (Senior House Official), Joe Sabatino (Pilot), Nadia Shazana (Jacy), Tyler Toles (Sick Boy), Alex Virzi (Adam #1), Mitchell Virzi (Adam #2), James K. Ward (Scientist), Michael Yama (Epidemiologist)
As the smallpox crisis continues, the CDC evacuates the whole hospital, but Carter, Abby, Chen, Pratt and Stan are placed under quarantine in the ER: Romano's arm gets severed by a helicopter blade as he and Kovac clash over transporting their patients; meanwhile, Corday is revealed to be working at her father's hospital in England, but her new colleagues are less than thrilled about her style of practicing medicine.