Derek takes Cristina out on a fishing trip; Teddy tries to help a patient who lacks the proper insurance to seek medical treatment.

A gunman opens fire at a local college, sending mass casualties to the hospital; Arizona's return is not warmly received.

Arizona tries to reconcile with Callie, who has big news to share; Teddy faces an important decision; the residents receive new interns; the Chief makes an announcement.

Meredith, looking to prove she is Chief Resident material, learns that anything and everything can happen in an hour's time when she steps up to run the ER for a night; meanwhile, Bailey sneaks off with Eli and gets into a little mischief, and everyone is surprised when the Chief's wife, Adele, shows up as one of the ER patients.

Richard is desperate for Derek and Meredith to accept Adele into their Alzheimer's trial, but will Adele stand in her own way? Meanwhile Mark and Arizona have conflicting ideas of the kind of baby shower Callie wants, Teddy is freaked out when Henry has another health complication -- leaving them both a little exposed -- and the competitiveness amongst the residents for the Chief Resident position is amplified when Richard receives the green light for his diabetes clinical trial.

A cataclysmic event leaves the doctors shaken; Callie, on the verge of a life-changing moment, starts imagining about the hospital and her friends.

Everyone pulls together to help Callie and baby Sofia on their painstaking journeys to recovery in hopes of allowing mother and baby to finally meet; the sudden death of a clinical trial patient has Derek proceeding with more caution while Meredith makes a calculated and risky move; Alex treats a cranky old rich woman dying of cancer and gets an idea for a grandiose endeavor that is sure to get him the Chief Resident spot; and Teddy is pleasantly surprised by the return of a familiar face.

Personal and work pressures are adding up and have Meredith visibly on edge, Owen conducts formal interviews for the Chief Resident position, Cristina grows increasingly defiant, Alex and Lucy's undefined relationship gets tested, and Jackson suddenly backs out of the Webber's diabetes trial.

News of Meredith's dishonesty in the clinical trial comes to light, leading to unanticipated consequences for her and others; Cristina finds herself in a compromising situation, which forces her to decide between her career and her relationship; Owen posts his decision for Chief Resident, and Teddy makes a surprising choice regarding her love life -- and her future.

News of Meredith and Derek's unsteady relationship raises a red flag for Zola's adoption counselor; Alex quickly realizes that he has become the outcast of the group after ratting out Meredith; and Cristina makes a tough decision regarding her unexpected pregnancy. Also, Chief Webber brings Henry in for a last minute surgery, alarming Teddy.

This is an historic episode that finally crosses a long-standing barrier.

The residents try their hardest to impress surgical great Catherine Avery-who also happens to be Jackson's mother-when she arrives at Seattle Grace to perform a groundbreaking transplant surgery; Arizona and Alex find their hands tied with a confidentiality issue when a familiar patient comes into the ER; meanwhile, a love-struck Teddy throws a dinner party for the couples, and Bailey makes a decision regarding her love life

Owen stresses teamwork and moves his leadership role over to the baseball field when he signs the doctors up for a baseball league, pitting them against their biggest competition, Seattle Presbyterian; Lexie tries to hide her jealous rage when she sees Mark with a new woman, but her emotions get the better of her; Alex fights to keep Zola at Seattle Grace after it is suggested that she be moved to another hospital due to a conflict of interest with Meredith and Derek; and Richard scolds Meredith and Bailey for their feud

The doctors become nostalgic when George O'Malley's mother, Louise, returns to Seattle Grace for medical help after a botched surgery at a neighboring hospital; the residents are inspired by a medical miracle when they witness a harvested heart that continues to beat outside the body; a new pediatric fellow excites Arizona and makes Alex feel threatened; Jackson lets his suspicions about Mark and Lexie interfere with his work; and Henry and Teddy have their first marital fight when he expresses interest in pursuing medical school

Teddy puts her husband’s life in the hands of her peers when Henry is sent to the OR for surgery and she’s called away on another case; Meredith and Derek receive the long-awaited news about Zola; Callie and Jackson come under fire when their patient experiences post-surgery complications; and Meredith and Alex are involved in a life-threatening situation when they travel to a neighboring hospital to tend to a sick newborn, on Grey’s Anatomy

A severe car crash involving an entire family results in an all-hands-on situation in the ER, as the eldest daughter is left to make the toughest decision of her life. Meanwhile Teddy, still in the dark about Henry's death, calls upon Cristina to help with her patient in the O.R., and Lexie finds herself working alongside Mark's new girlfriend, Julia, during an eye surgery.