We opened on Izzie talking about how you never know the biggest days of your life are going to be the biggest. "It's the regular days, the ones that started out normal. Those are the days that end up the biggest." She said, "Today was the wedding, and it was beautiful."
Meredith and Derek stood in a big church and said, "We're getting married tonight."
Cristina was crabby because she slept in the on-call room so Callie could have the apartment to herself for a night with Arizona. But Callie slept at the hospital, too.
Izzie was excited about the wedding and all the plans she'd made using Derek's credit card. Izzie got permission from Bailey to go to the wedding and she wanted high-fives from Alex and Bailey. She played excited, but when they left, she realized her hair was starting to fall out, and she saw Denny.
"I know what this means, you being here," Izzie said. "I'm still going to the wedding."
It was a slow day in the E.R. until the phone rang. "Incoming!" Callie shouted.
Cristina and Hunt talked about how it was going with his shrink. The shrink told him the ceiling fan above Cristina's bed was the trigger of his unintentional attack on Cristina, because it reminded him of a helicopter.
Patients started pouring in from a massive accident in which a semi-truck jackknifed and swerved across four lanes. Callie, who was running the E.R., realized there were eight kids piled in an SUV on their way to graduation.
One girl wanted to know if her friend, David, was OK. He was to be the first in his family to graduate from college. She was the valedictorian and was supposed to give a speech. Another guy said he couldn't find his girlfriend in the wreckage.
One more guy was being treated by George when he suddenly coded.
Izzie paged Derek and asked him about details about the caterer's check and writing his vows. She said that wasn't why she paged him. She told Derek that Denny was back and she told him to order another MRI.
Derek had the chief give Meredith a colon for their wedding day -- a solo surgery.
Callie was stressed while trying to run the busy E.R. and Arizona wanted to talk about what was wrong with Callie on their date the night before. The guy George was trying to succumbed after 20 minutes and Hunt told George to let him go and save the ones he could still save. George ask Lexie to call the kid's time of death.
Izzie told Bailey she knew she had a tumor because she was seeing Denny. Bailey said, "Tell Denny Duquette I said, 'Go Home.'"
The valedictorian girl told Alex she regretted not going through with a relationship with her friend David, and, staring death in the face, she wondered if she should have done it when she had the chance.
Another of the patients told Cristina that he would have to return home the day after graduation, meaning he wouldn't be able to stay with the girl he loved. He knew they weren't supposed to be together -- like Cristina and Hunt -- but nothing could keep them apart.
George and Lexie started treating a new kid from the car accident, Pete (with red sneakers). Lexie remembered one of his friends, David -- the one who just died -- wanted them to find Pete, the one with the red sneakers. Pete wasn't going to make it, either.
The chief scrubbed in on Meredith's colectomy and was giving just a bit too much of a watchful eye.
Callie was clearly trying to avoid Arizona by choosing to work the E.R. on her day off. Sloan said, "You should just tell her."
George told Hunt that another one of his patients died, and he should pull him from duty. "I'm in over my head," George said. Hunt told him to look for another patient.
Izzie, Bailey and Derek tried to figure out what was wrong. The MRI was clean, but Izzie was still convinced there was a tumor because of her hallucinations coming back. Derek wondered whether they could induce a hallucination to try to figure out which part of the brain was causing them.
Another patient came in, the girl with the pink sweater -- the girlfriend of the guy who was going to have to move back home. She died immediately, and her boyfriend saw it and coded.
Arizona yelled at Callie about the way she was treating her, then suggested, "Maybe it's over."
"Maybe it is," Callie said.
The chief was watching every move Meredith made in her surgery, groaning each time she did anything slightly different than he would have.
Bailey and Derek hooked Izzie up to monitors to track her brain activity and try to figure out where the hallucinations were coming from.
Becca, the valedictorian, asked Alex to tell her how her friends were doing. "I can take it," she said. He didn't tell her, but asked her to tell him her graduation speech, because she worked hard on it. As she told him the speech, a montage of her friends' being closed up in body bags. First one, then another, and another, and another. Meanwhile, Meredith worked on her first solo, Izzie tried to conjure Denny, and another friend of Becca's was zipped up. Cristina tried to save another of Becca's friends, and lost him.
Izzie asked Denny to come to her and he finally appeared. Derek pinpointed the spot on her brain where the activity was coming from.
Derek spotted the tumor on Izzie's other temporal lobe. Bailey wondered how he was going to get to it. Derek didn't respond, but said, "I wish there was something I could do for her."
Bailey finally said, "I know something you can do."
Arizona yelled at Callie again, and Callie told her she was broke, and that was why she was acting strangely the night before. Callie said she was working at the E.R. because she needed the money. Arizona said they could stay in and eat sandwiches. Then they decided to get pizza instead.
The chief told Meredith he'd been reading her mother's journal and wished Ellis had been there to see Meredith's first surgery.
Meredith thanked him for the surgery. She talked about getting dressed for the wedding, but Derek said, "What do you say we make this day even better?"
Hunt told George he did good work, but George didn't believe him because every patient he touched died. Hunt said not all of them died, and the one he was able to save was alive because Hunt had George on his team. Trauma is a team sport, Hunt told George.
Alex sat outside the hospital lamenting the fact that they'd found another tumor in Izzie's brain. Alex cried as he told Meredith, "I think she's going to die on me." He said he was supposed to be getting ready for a wedding.
"Well, that's what I came to talk to you about," Meredith told him.
Lexie went to take care of Becca, who breathed on her own. Lexie cried just a little.
Izzie was getting impatient as Bailey was all dressed up and eager to get to Meredith's wedding. Finally, Meredith came running in with the wedding dress in hand. Meredith said, "This dress isn't for me." Izzie thought Meredith was freaking out about getting married, but then Alex came in wearing a tux and said, "She's right," meaning the dress wasn't for Meredith. Izzie smiled.
Izzie decided to have Cristina be her maid of honor because Alex needed a best man and asked for Meredith. Cristina asked if she was just leftovers.
"You tried to save my life," Izzie said. "That makes you honorable."
Izzie struggled to make the walk down the aisle on her own, but George jumped up and walked her the rest of the way.
Izzie realized they hadn't written any vows. Alex said there was something he wanted to say.
"Today is where my life begins," Alex started, borrowing heavily from Becca's valedictorian speech.
He ended with, "Today, Izzie Stevens, our life together begins. And I can't wait to get started."
They were pronounced husband and wife, and they kissed.
"You never know the biggest day of your life is the biggest day," Izzie said in a voiceover. "Not until it's happening."
Alex put a tired Izzie back in her hospital bed and took her hairpiece out -- and hair came with it. She worried about losing her hair and Alex told her she'd be beautiful without it.
As Izzie talked about the biggest day of her life, we saw Callie and Arizona in bed, Cristina tearing down her ceiling fan, and Derek and Meredith celebrating their "best non-wedding day ever."
It turned out Izzie's voiceover about the biggest day of your life was her talking to Denny. She wore a scarf on her head, but when Alex walked in he told her she didn't it and looked beautiful without it.
"He's right," Denny said.
Izzie asked Denny to leave. She wanted to be alone with her husband.
Alex sat on her bed, looked at Izzie's bald head and said, "My wife is hot."