Kate Walsh Returns to 'Grey's Anatomy' : A Look Back at Addison Montgomery's Complicated Past

With Kate Walsh reprising her role as Dr. Addison Montgomery on Thursday's episode of Grey's Anatomy, what better way to prepare for her return than with a trip down memory lane?

Here, we take a look at Addison's complicated history on Grey's — including the love triangle that launched the hit ABC medical drama, her former romantic entanglements and career moves — as well as where she left off when her spinoff, Private Practice, which aired for six seasons, concluded in 2013.

Addison, a neonatal surgeon, first appeared on Grey's during the season 1 finale in 2005 when she caught then-husband Derek Shepherd (Patrick Dempsey) and Meredith Grey (Ellen Pompeo) off guard. (Who could forget her famous line: "You must be the woman who's been screwing my husband"? Yikes!)

While Meredith and Derek's romance was heating up after some ups and downs during the first season, their relationship came to a quick halt after Derek's estranged wife (Addison) unexpectedly showed up to then-Seattle Grace Hospital. Later we find out that Addison and Derek's relationship was strained due to her infidelity after she slept with Derek's BFF Mark Sloan (Eric Dane).

In the second season, Addison and Derek tried desperately to reconnect and give their marriage a second chance — though it was obvious Derek was still in love with Meredith.

Enter McSteamy.

Trying to ignore the signs of Derek's affection for Meredith, Addison was hit with another curveball when Mark (aka McSteamy) arrived at Seattle Grace in hopes of winning Addison back. This is where things got messy.

When Derek found out that Meredith had moved on, he slept with Addison (the first time since her return). But then, once Addison found out that Derek was actually in love with Meredith and not just rebounding, she slept with Mark ... again, creating an odd love square?

Once Addison realized her marriage to Derek was officially over, she drowned her sorrows in muffins and booze at Joe's (the bar that all the Grey Sloan doctors frequent after a bad day) and eventually had a small fling with Alex Karev (Justin Chambers) in season 3.

Addison eventually decided it was time for a change, both personally and professionally. After realizing she wanted to become a mother, Addison left Seattle to visit her friend and fertility specialist, Naomi — and so began Addison's transition to the Grey's spinoff Private Practice.

Addison moved to Los Angeles to start a new life, occasionally returning to Seattle through seasons 4-8 of Grey's.

Addison eventually became a mother after adopting her son Henry, and she married Jake (Benjamin Bratt) in the Private Practice series finale in 2013.

Last month, Walsh announced her return for Grey's season 18 in a video message on the Grey's Anatomy social media pages.

"Well, well, well, would you look who it is. That's right, my loves, Dr. Addison Montgomery is coming back to Grey Sloan Memorial Hospital, and I'm so excited to be home again joining Shonda, Ellen and the rest of the incredible cast," Walsh, 53, said in the video, referring to series creator Shonda Rhimes and star Ellen Pompeo.

Last week, Grey's fans got their first look at Walsh's return to the ABC medical drama in a sneak peek at Thursday's episode, titled "Hotter Than Hell."

In the clip, Walsh's Addison comes to the hospital to assist Dr. Richard Webber (James Pickens Jr.) with his latest crop of interns.

RELATED VIDEO: Grey's Anatomy: Kate Walsh Arrives at Grey Sloan in Preview for Next Week

"I'm Dr. Addison Montgomery, and you must be the group that's been screwing up the program," she tells the interns.

As Addison is operating on a patient, the air conditioning goes out across the hospital. When Richard comes in to break the news, Addison tells him: "I can handle a little heat."

However, the heat isn't the only problem. The HVAC system controls the air filtration system in the hospital, meaning the patient needs to be closed as soon as possible to prevent infection, Richard says.

"Find a way to double your speed," Richard insists.

"Find me a set of hands as experienced at microvascular anastomoses of the abdomen," she retorts. She pauses before adding, "Get me Meredith Grey."

In the eight years since Walsh left the franchise, much has changed. Only three original, season 1 players remain: Pompeo, Pickens and Chandra Wilson. But one other cast member will be familiar to Addison, as her former sister-in-law Dr. Amelia Shepherd (Caterina Scorsone) began her journey on Private Practice before moving to Seattle following its final season.

Speaking with PEOPLE last week, Pompeo said she was thrilled to reunite with another original cast member for the upcoming episode.

"Kate's so fun!" Pompeo, 51, said of the actress. "She moved to Australia, so I really haven't seen her or hung out with her. I have so much affection and love for the original cast. We all went through something that only we can understand. So it's always just really fun and very emotional, to be honest. When any of the original cast members get together, we always start crying, and we're like, 'Why are we crying?' "

Grey's Anatomy airs Thursdays at 9 p.m. ET/PT on ABC.