The residents of Seattle Grace and the newly arrived residents from Mercy West vie for surgeries and their careers as day one of the hospital merger begins. Recovering cancer patient Izzie (Katherine Heigl) experiences the brunt of her new surgical competition.

"There should be a couple of days when they act as guests before they put their feet up on the coffee table," Izzie complains to a bedridden Meredith (Ellen Pompeo), Cristina (Sandra Oh) and Alex (Justin Chambers).

"We are not some stupid hosts. They can't invade. They can't attach themselves to our faces and then when we are eating spaghetti burst out of our chests and skitter across the floor," says Meredith, who has been watching too much TV while recovering from donating a portion of her liver to her father. "We own this hospital … and we will hold this hospital with our last gasping breath."

Meanwhile, Arizona (Jessica Capshaw) stands by an enraged Callie (Sara Ramirez) when her father, Mr. Torres (Hector Elizondo), employs a priest to help convince her to date men.

"The man flew 3000 miles to make me straight," and indignant Callie tells her lesbian lover. "With a priest! I am lucky they didn't march into the ER swinging incense."

But Arizona urges her to talk to her father, saying, "You are the one who changed the game. You dated men your whole life. You even married one."

The "Invasion" episode of "Grey's Anatomy" airs Thursday, Oct. 15 at 9 p.m. on ABC.