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Ambush is the first episode of the fourth season of ER, and the seventy-first episode overall.
Guest Stars: Ellen Crawford (Lydia), Tracy Vilar (Doris), Lisa Edelstein (Aggie Orton), Brent Jennings (Nat), Peter Birkenhead, Jessie Nelson, John Pleshette (Mr. Schoenberger), Darnell Williams (Mr. Williams), Nicki Micheaux, Justina Machado, Nick Offerman, Summer Phoenix (Petra), Sherman Howard, Jesse Borrego (HIV-Positive Patient), Gerald S. O'Loughlin (George), Curt Bisquera, William H. Macy (Dr. David Morganstern), Conni Marie Brazelton (Conni), Laura Ceron (Chuny), Deezer D. (Malik), Lily Mariye (Lily), Kristin Minter (Randi), Emily Wagner (Pickman), Monte Russell (Zadro), Lynn A. Henderson (Olbes), J.P. Hubbell (Audia), Silas Weir Mitchell (Luis), Darion Basco (Justin Chang), Francois Chau (Neurosurgeon), Matthew Fishman (Todd Lebeck), Audrey Gelfand (Weeping Woman), Googy Gress (Head Lac), John Hawkes (P.A.), David Stifel (Boz), Carlos Alvarado (Gang Member)
A documentary video crew follows Mark and the rest of the ER shift through a typical day at County General; new surgical resident Elizabeth Corday starts her first day at County; Morgenstern suffers from a heart attack; Mark treats a man who has been left paralyzed after attempting to save a teenage boy from being assaulted by gang members.