Weaver returns to the emergency room after a self-imposed sabbatical of three weeks, wondering how her newfound lesbian identity will affect her workplace relationships. Having purposely allowed a mass murderer to die, Greene must keep the secret amidst the scrutiny of his superiors.

Carter is overwhelmed by patients from a rock concert riot. Benton's former girlfriend is killed in a car accident. Newly promoted to chief resident, Chen panics and misdiagnoses a patient. Kovac angers Abby by flirting with a waitress.

Greene's daughter comes to live with him and Corday. Carter and Abby become more intimate. Chen must prove her skills while saving a baby. Weaver terminates Malucci after witnessing him having sex in an ambulance.

Susan è tornata a Chicago e vorrebbe lavorare ancora al pronto soccorso. Nel mentre, Jing Mei viene sospesa per un mese. Luka è sorpreso quando una bellissima cameriera francese arriva al pronto soccorso e chiede di lui.

Lewis faces a hectic first day back at County General. Benton and Roger become frantic when they can't locate Reese. Carter's grandmother is brought to the emergency room. Carter and Abby deal with inept medical students.

Corday's ordeal, caused by the recent infection-related deaths of several of her patients, worsens when she is closely observed by a stern taskmaster. Corday is outraged when a county investigator accuses her of deliberately euthanizing the patients, who all happened to be elderly.

A new medical student, Michael Gallant, arrives in the ER. He's supposed to be working under Dr. Carter's supervision but his grandmother is unwell. Elizabeth Corday is convinced that other doctors are refusing to work with as a result of the mysterious deaths.

When a torrential rainstorm pounds Chicago, patients deluge the emergency room while Weaver speeds to an accident site. There, she finds a pregnant woman, Vicki lying injured inside a wrecked ambulance while live power lines spark. Fourth-year student Michael Gallant makes a dangerous decision in order to help.

Benton e la sua avvocatessa Janice Prager intraprendono una dura azione legale in tribunale per togliere l'affidamento del figlio a Roger. Intanto, mentre Corday aspetta che un vedovo si decida a dare il permesso per l'asportazione degli organi della moglie cerebrolesa, Greene e Lewis si occupano di un pugile adolescente con gravi danni al cervello.

Benton commits perjury on the witness stand in order to gain custody of Reese. When Romano refuses to give him a more even schedule, Peter resigns and seeks a job at the clinic where Cleo is newly employed. Benton is granted custody of Reese and he and Cleo decide to move to the suburbs together. Meanwhile, Carter's father visits for Christmas with the news that he's getting divorced, Weaver and Sandy Lopez finally go on a date, and Abby catches Nicole in the maternity ward, recovering from an abortion.

Abby helps a distraught boy whose mother died of a stroke; a man with schizophrenia, who once stabbed Carter and killed a doctor checks into the ER.

An accidental overdose brings Drs. Green and Corday's baby into the ER.

Greene confronts Corday when she orders Rachel out of their home; Abby tries to protect a battered neighbor who refuses to press charges against her husband.

After Dr. Corday moves out, Dr. Green fights loneliness and pain by immersing himself into his work. Dr. Chen discovers that a comatose teen is ready to give birth.

Susan rientra a casa dopo aver passato la notte ad occuparsi di Mark e trova Carter sotto casa ad aspettarla. Lui sospetta che ci sia del tenero tra i due e la cosa sembra infastidirlo, ma lei smentisce senza però rivelargli la verità sullo stato di salute dell'amico. Contemporaneamente Abby si risveglia a casa di Luka dove in attesa di trovare un nuovo appartamento passa la notte sul divano.

All'arrivo all'ospedale i primi pazienti sono una coppia BDSM ed a prestare i soccorsi sono proprio Carter, Susan, Luka ed Abby. Il caso si risolve in fretta, ma lascia grandi strascichi di ilarità, soprattutto fra le ragazze che imprudentemente aprono una sacca abbandonata dalla Mistress scoprendo tutta una serie di gadget erotici. L'arrivo della Dr.ssa Weaver interrompe bruscamente i giochi ed i quattro insieme al giovane studente Gallant vengono spediti per punizione ad una lezione di educazione sessuale. Il ritardo del professore consente ai medici di approfondire la conoscenza l'uno dell'altra, i trascorsi di vita e le gelosie tra le due coppie di "quasi amici".

Alla fine dell'episodio Susan lascia andare Carter perché capisce che tra loro non c'è quella scintilla e lo incoraggia a farsi avanti con Abby. Si scopre inoltre che il responsabile dello scherzo all'armadietto della Weaver è proprio lo stesso Carter.

Drs. Corday and Greene are awkward when working together in the ER. Dr. Weaver treats victims of an intense fire that may take the life of a dear friend.

As Dr. Greene's brain tumor begins affecting his coordination, he becomes concerned that his fate will be similar to the terminally ill patients he treats.

When Dr. Lewis gets a hysterical call from her niece, Susie, she hurries to New York to search for the child and her sister Chloe.

The crossover continues on Third Watch S03E19 Unleashed

Dr. Carter reads a letter from terminally ill Dr. Green to the ER staff. Dr. Weaver lashes out at a mentally challenged patient. A terminal, homeless man continues asking for Dr. Green.

Dr. Greene's final days are spent in Hawaii with Rachel, teaching her to drive and surf. When he suffers a seizure, Elizabeth is called. Mark refuses to go home. He manages to make amends with his daughter before his eventual death.

Just when the ER is understaffed and swamped, two children who were brought in turn out to have full blown smallpox. The entire ER must be locked down. The staff needs to improvise while they battle not only the disease but also frayed tempers.