Tough calls: Derek was approached by Isaac, a hospital lab tech with a tumor wrapped around his spinal cord. Derek first called the tumor inoperable, because it would require cutting the spinal cord to remove it. Isaac pressed Derek to reconsider.
The other doctors — even Bailey — found the tumor so fascinating that they sort of wanted Derek to do it, but the Chief ordered him not to, for reasons including the cost. So naturally, Derek scheduled the surgery, but concealed it from the Chief.
Lexie and Avery scrubbed in, and Lexie even put on an adult diaper so she wouldn't have to go to the bathroom. Isaac pressed Derek to cut his spinal cord if necessary, arguing that there is always a way to do things that seem impossible.
Try, try again: Eight hours into surgery with Derek unsure of what to do, Bailey dropped in to say that the Chief was nearby, so Derek needed to get a move on. (This was just as Lexie gave up on the diaper and took a bathroom break.) Shortly thereafter, the Chief arrived and ordered Derek to close Isaac up — which Derek did. But when he broke the news to Isaac later that he couldn't figure out how to do the surgery, Isaac was confident Derek would try again. And after reviewing the situation with Meredith and formulating a plan, Derek decided he would.
Derek told the Chief that the surgery would be simple because he'd cut the spinal cord, so the Chief relented. Derek then revealed to Lexie and Avery that he wasn't actually going to cut the cord. He had a plan for a long, costly surgery to save Isaac's legs. Lexie vowed that this time, she would use the diaper, and she brought in Mark to give Derek support. Derek threw up part of the way through surgery, but Lexie got him back on track.
Mixed emotions: Once again, the Chief figured out what was going on and tried to storm the OR, but he was stopped in his tracks by Arizona, who pointed out that disrupting the surgery could hurt the patient. Derek eventually had to decide which of two vessels to cut — like the two bomb wires in a spy movie. With all the other doctors observing, he finally picked one, and it turned out to be the right one. Isaac was grateful; Derek felt like a god — happy ending. Except that the Chief, who apparently expects people to tell him the truth instead of repeatedly lying, fired Derek. Derek sent the Chief home to "sleep on it."
The Alex update: Alex continued to bicker with Reed. He was grouchy over Izzie's absence, but was keeping the faith that she'd return. Izzie was scheduled for a session of cancer treatment, and Reed persuaded Alex to go down and try to meet her. But Izzie never showed, leaving Alex devastated and crying on Reed — which will mean something, in all likelihood.