Just when you think you've got Grey's Anatomy all figured out, series creator Shonda Rhimes and her amazing writing staff throw you a curve. Case in point: Thursday's story line between primetime's leading lesbian ladies, Dr. Callie Torres (Sara Ramirez) and Dr. Arizona Robbins (Jessica Capshaw).

The episode, titled "Invest in Love," revolved around Seattle Grace/Mercy West's continuing misplaced financial priorities that were seemingly resolved thanks to the tender care that Arizona was providing to Wallace - a 10-year-old with intestinal issues. The boy's parents generously announce that to celebrate Wallace's 11th birthday - which happens to fall on Arizona's birthday - that they plan to donate $25 million to the hospital for research to help find a cure for the rare intestinal disease.

When Callie hears the news from Lexie (Chyler Leigh) about the donation and that it falls on Arizona's birthday, she's stunned that she had no idea when her girlfriend's birthday was and has absolutely no clue what to get her or how to celebrate.

As the episode progresses, Wallace's condition takes a turn for the worse and Arizona, at the boy's parents' insistence - and with Chief Webber's (James Pickens Jr.) encouragement - that the out doc perform a dangerous surgery to help prolong the youngster's life.

That's when things get interesting. Arizona begins doubting her surgical skills and turns to Callie for reassurance that she's a good doctor. Callie does the trick - in what wasn't a convincing scene at all - and Wallace goes under the knife.

Cut to a shattered Arizona making her way to Callie's apartment who is welcomed in by a surprise party. Arizona, who previously confessed that she's not really into birthdays, begins to cry and isn't in any sort of mood to mark the occasion.

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She storms out and Callie follows where Arizona unloads that she was paging Callie after losing Wallace to post-surgery complications and she wasn't there. A frustrated and hurt Arizona cites that she's been there for Callie through everything and she really needed her.

Paired with Callie's seemingly flat reassurance, things don't look good for "Calzona."

And that's when Grey's does what it does best and throws you completely off guard: Arizona returns to Callie's place to find her girl asleep on the couch waiting up for her, clad in sexy lingerie, gifts at the ready. An overcompensating Callie snaps awake and is ready to turn Arizona's frown upside down. Arizona's reaction: The those three famous little words that can change everything: "I love you."

Fade to credits, but before the audio is cut, Callie whispers it right back to Arizona.

A sweet ending to an amazing episode that illustrated just how great an add Jessica Capshaw is to the show: A doctor who finds the balance between having heart and standing up for what's right (no offense, Izzy).

So how did you feel about Calzona's "I love yous"? Think Grey's will give us a lesbian wedding this season?