Cristina, back from her honeymoon trip, searches in vain for Burke; Meredith, Cristina, Izzie and Alex spend their first day as residents with their own groups of interns

The residents treat the victims of a massive explosion; Mama Burke returns to collect her son's things; Lexie tries to have a heart-to-heart with unwilling Meredith.

George's skill level dazzles the new interns; Sloan and Richard attempt a radical new surgery; George tells Callie of his past indiscretion with Izzie.

The Chief's wife brings their niece to the hospital; Izzie reveals her feelings about George to a mutual friend; Derek shows Cristina how to be a better teacher; Norman misdiagnoses Meredith's patient.

Alex receives an unexpected visit from a former patient; Meredith thinks her mother's ashes are haunting her; a surgeon Cristina admires snubs her.

The chief has a gentlemen's evening with his fellow surgeons; Cristina and Izzie find themselves competing for the same surgeries; Meredith tends a sky-diving survivor.

George and Izzie test their romantic chemistry; Derek asks for more than a physical connection with Meredith; Mark develops a crush on a fellow surgeon; Callie finds her role as chief resident jeopardized.

Teenagers fill the ER after a school-bus crash; Bailey treats a patient who was her high-school crush; Derek goes on a date with a fellow doctor.

An ambulance crash endangers the lives of the paramedics involved; Bailey treats a patient who refuses her help; Ava returns for Alex and meets his other girlfriend.

Meredith works to save the life of a paramedic trapped in an ambulance; Lexie holds a patient's life in her hands; Derek must rely on a nurse's help to save a patient; Bailey struggles to balance her personal life with work.

Bailey's family faces a medical trauma just as her marriage falls apart; George's mother makes a surprise visit; Derek must deal with the aftermath of his kiss with Rose.

Six weeks after Derek and Meredith ended their relationship, Meredith, Alex, Izzie and Cristina compete in a surgical contest, with Bailey serving as judge.

Addison returns to Seattle Grace to perform an operation and is surprised by the changes in her former co-workers; Derek and Meredith's clinical trial has its first patient.

The nurses' complaints about Mark Sloan's trysts result in a date-and-tell policy; Cristina is forced to face her feelings for Burke; Meredith and Derek admit a veteran into their clinical trial.

Dr. Wyatt refuses to let Meredith give up on therapy; Alex learns the truth about Rebecca's medical condition; the Chief insists Erica perform a risky surgery on his mentor.

Meredith and Derek get one last shot at success in their clinical trial; the surgeons work together to free a boy from a block of cement; Lexie uncovers information about George's status as an intern.

Meredith and Derek get one last shot at success in their clinical trial; the surgeons work together to free a boy from a block of cement; Lexie uncovers information about George's status as an intern.