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.

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

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.

A gas tanker explodes on a Seattle highway, causing multiple injuries, including paramedic Matthew. Meredith and Jo help a frantic mother whose child is suffering from a mysterious illness. Owen bonds with a young boy whose parents are in critical condition. Meanwhile true colors are revealed when Alex is forced to work with Jo's boyfriend on an ailing patient.

The Grey Sloan Memorial Hospital doctors are faced with the devastation left by the storm. A giant mudslide in Seattle injures a group of first responders and citizens, causing the already short handed ER to spring into action. Meanwhile, Meredith is faced with a tough decision that will affect the life of a loved one, Callie is rocked by the reality of Arizona’s infidelity, and Richard’s life is in extreme danger.

Costumed patients create a challenge for the doctors; Derek and Ben work on a new surgical technique; Richard asks a resident to perform his next surgery, angering Bailey.

Owen tries to get Callie to help Veterans Hospital patients with the robotic limb lab. Alex and Meredith's friendship makes Jo jealous. Alex and Bailey prepare to go in front of the board.

Richard, Jackson, April and Catherine tackle a grueling trauma case intensified by hospital politics. Amelia finally faces her feelings about Owen, and Meredith gets caught between Nathan and Alex over a patient.

Diane Pierce returns to Grey Sloan, but Maggie is still in the dark as to why she’s really there. Owen and Amelia hash out their problems as they work a trauma case together, and Bailey tries to mend her relationship with Richard.

Harper Avery arrives at Grey Sloan, putting Bailey on edge. A familiar face from Meredith’s past returns as a patient, and Amelia tries to manage a secret.

A young boy is admitted to Grey Sloan Memorial and his case has a profound impact on the doctors. Meanwhile, April is faced with a surprising patient, and Jo continues to deal with her estranged husband.

April is in charge of the new Grey Sloan Surgical Innovation Contest, and the doctors are eager to start their projects. Meanwhile, Catherine’s old friend has a shocking idea for Catherine and Jackson; and Meredith treats a returning patient who inspires her project.

One of Grey Sloan Memorial’s own is seriously injured, making the team reflect on what is truly important to them. Meanwhile, Nicole Herman pays a visit to Grey Sloan and talks to Arizona about an exciting opportunity.

Meredith works on a patient who happens to specialize in matchmaking, and a seemingly drunk patient pushes Richard's buttons and gives him a medical mystery to solve. Meanwhile, Maggie tries to reach Teddy after bearing the burden of her secret becomes overwhelming as she struggles with issues in her own personal life.

Meredith breaks a hospital record; the doctors are sent scrambling in the wake of a massive overdose within the community; Jackson wants to take Maggie camping; Levi and Nico's relationship continues to develop.

Ties are strained as Andrew and Carina confront an unforgiving family history. Meanwhile, Teddy and Owen work together on a married couple who is expecting, and Amelia attends a conference for alternative pain relief.