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Mars Attacks is the third episode of the seventh season of ER, and the one hundred thirty-ninth episode overall.
Starring: Anthony Edwards (Dr. Mark Greene), Noah Wyle (Dr. John Carter), Laura Innes (Dr. Kerry Weaver), Alex Kingston (Dr. Elizabeth Corday), Paul McCrane (Dr. Robert Romano), Goran Visnjic (Dr. Luka Kovac), Maura Tierney (Nurse Abby Lockhart), Michael Michele (Dr. Cleo Finch), Erik Palladino (Dr. Dave Malucci), Ming-Na (Dr. Jing-Mei Chen)
Guest Starring: Ellen Crawford (Lydia Wright), Troy Evans (Frank), Alex D. Linz (Dennis), Laura Cerón (Chuny Marquez), Lily Mariye (Lily Jarvik), Kyle Richards (Dori), Jorge Luis Abreu (), Rebecca Balding (Mrs. Garvey), Christine Cavanaugh (Gloria), Kevin Durand (Mr. Mooney), Jamie Marsh (Mr. Kamatovik)
Co-Starring: Nynno Ahli (Security Guard), Michelle Bonilla (Harms), Duffy Epstein (Griffin), Adam G. (Pelvis Man), Steve Hanneman (Patient), Bradley James (SWAT Guy), Brian Lester (Dumar), Aaron Pruner (Alien Boy), Devon Reeves (Moira), Charlie Stewart (Albert), Eric Stonestreet (Willie), Toshi Toda (Japanese Businessman #1), Craig Tsuyumine (Japanese Businessman #2), Ludo Vika (Lupe), Emily Wagner (Pickman), Claudia Michelle Wallace (Parking Attendant), Christopher Warren (Boyfriend), Matthew Watkins (Reese)
Carter returns to work after completing his drug rehab and cares for a handcapped boy with a bladder infection; the ER is filled with patients after a beam collapses during a sci-fi convention and Corday talks Mark through a complicated procedure on one of the patients; Romano fires Benton over the EMTLA violation; Weaver has Abby return to the ER to work full-time as a nurse; Frank annoys the ER staff when he runs background checks on the patients.