The fifth-year residents return for the first day of a year that will make or break their careers: Meredith faces the consequences of tampering with Derek's clinical trial and is terminated at the hospital; April tries to step up to the plate as Chief Resident in the wake of a giant sinkhole in the middle of Seattle; and Cristina and Owen are still at odds over their drastically different feelings for their unborn child. 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 chief makes a decision that shocks everyone at Seattle Grace; Owen and Cristina attempt to find normalcy; April struggles in her new role; the 5th-year residents receive their first solo surgery assignments.

A stampede at a comic book convention leads to an influx of colorful characters in the ER; Mark continues to make life difficult for Jackson; Ben returns and realizes Bailey has moved on.

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

After being blackballed by most of the surgeons, Meredith uses Lexie to spark Derek's interest in her patient's rare brain tumor case and, in secret, gives guidance to April with Bailey's clinical trial; Callie's wish for Arizona and Mark to form a friendship comes to fruition and she realizes it's more irritating than helpful; Alex and Teddy research alternative surgeries for a very superficial patient; and Callie and Cristina struggle with a very difficult spinal surgery.

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

The doctors split into teams for a risky surgery involving conjoined twins; Bailey recruits Meredith to be a buffer between her and Ben when he puts pressure on moving their relationship to a more serious level; Richard teaches Alex a tough lesson in the OR; meanwhile Teddy questions Cristina about what exactly happened to Henry during his surgery.

Richard performs a monumental surgery in his career; Derek and Lexie attempt to work miracles for a patient with an inoperable tumor

As Meredith puts Zola to bed and falls asleep, she begins to wonder -- what if her mother had never had Alzheimer's and she'd had loving, supportive parents? The reverberations of a happy Meredith Grey change the world of Seattle Grace as we know it. What if she had never met Derek in that bar and he had never separated from Addison? What if Callie and Owen had become a couple long before she met Arizona? And what if Bailey never evolved from the meek intern she once was?

In the spirit of Valentine's Day, Meredith and Derek try to have a night to themselves when Lexie offers to watch Zola; Callie hands babysitting duties over to Mark so that she can surprise Arizona with a romantic evening; and Ben proves to Bailey that he knows her better than she thinks. Meanwhile, the ER fills with romantics, including a 10-year-old girl who has an allergic reaction to chocolates from her valentine and a man who is hit by a car while chasing down his frustrated girlfriend.

Amelia arrives at Seattle Grace to beg Derek to help with the gliosarcoma case; Cristina and Owen have heated arguments during marriage counseling; Alex realizes he needs to improve his people skills in order to be a good doctor.

As Adele's condition continues to deteriorate, Richard considers altering their living situation; an explosion at a local coffee shop results in a busy day in the ER for the doctors; Callie takes Meredith under her wing by helping her study for her boards; Lexie joins Arizona and Alex in peds, as they continue to care for intern Morgan's premature son; and when Jackson snaps at a patient, Mark encourages him to find ways to relieve his stress. Meanwhile, Cristina lets her suspicions get the best of her.

Derek encourages a hesitant Meredith to work with him again in neurology; Jackson's mother, Catherine Avery, returns to Seattle Grace with a urology fellow and develops an interest in Richard; and Cristina becomes increasingly suspicious of Owen's every move. Meanwhile, Alex tries to deny the fact that Morgan may be falling for him.

A lion breaks loose in Seattle, leaving a couple's lives and relationship at risk; Lexie overhears Mark's discussion about moving in with Julia; Teddy begins to take steps towards coming to terms with Henry's death; Callie grills Arizona on her past lovers; and Alex requests to be taken off Morgan's preemie case when she becomes too dependent on him. Meanwhile, Meredith tries to be a support for Cristina as the tension between her and Owen hits a boiling point.

A devastated Cristina presses Owen to confess the intimate details of his affair; Mark takes his new authority very seriously when he steps in as chief for the day and butts heads with Richard over a patient's liver transplant; and Callie deems Meredith ready for the boards, while the rest of the residents continue to stress about the exam. Meanwhile, Callie, Arizona and Bailey plan a much needed ladies night for Teddy.

The doctors work on a Jane Doe, who turns out to be the subject of a case that gained national interest; Cristina proves to be the highest in demand as the residents begin their interviews for post-residency positions at prospective hospitals; and Richard is faced with an unsettling realization when he visits Adele at Rose Ridge.

After eight years of practice, the residents head to San Francisco for their oral boards; sparks fly between Catherine and Richard when they bump into each other at the hotel; Bailey intervenes after Owen and Teddy get into a heated argument about their young car crash patient; and Mark tries to recruit Lexie for one of his cases. Meanwhile, Alex feels guilty for being away from the hospital when Tommy's health begins to fail.

The residents agonize over their oral boards, reliving every answer they gave during their exams; Arizona's close childhood friend comes to Seattle Grace for medical help; the doctors work on a patient who is missing one third of his skull; and Julia asks Mark to start a family with her. Meanwhile Catherine tells Richard that one of his residents has failed.

The residents determine their future as they make their final decisions regarding post residency positions; Mark admits to Derek that he is torn between his love for Lexie and the fact that Julia can give him everything he's ever wanted; Ben plans something extra special for Bailey, but she is preoccupied with her work; and Arizona encourages her childhood friend, Nick, to have surgery. Meanwhile a couple on their third honeymoon come to the hospital complaining of mysterious symptoms.

Faced with a life threatening situation, the doctors must fight to stay alive while trying to save the lives of their peers; Bailey and Ben make a decision regarding their relationship; and Teddy is presented with a tempting offer. Meanwhile, Richard plans a special dinner for the residents.