On Thursday's The Big Bang Theory, Leonard (Johnny Galecki) and Sheldon (Jim Parsons) — now that they're living in separate apartments — divvied up the possessions on which they share ownership. Things quickly escalated into all-out-war, though, when Leonard struck back against Leonard's selfishness... And it took Back to the Future's Christopher Lloyd to make peace.

It all starts when Amy (Mayim Bialik) brings a hideous painting of her and Penny (Kaley Cuoco) back from Penny's old apartment to Leonard's place, since Amy thought Penny would want it. Penny doesn't and tries to convince Amy to keep it, arguing that it's both of their painting and it's Amy's turn to hold on to it.

Inspired, Sheldon suggests that he and Leonard divide up their shared possessions. Sheldon gets Longclaw, their piece of Game of Thrones memorabilia, their one-of-a-kind Mr. Spock cuckoo clock and all of their board games, while Leonard claims the official flag of their apartment.

This incenses flag-obsessed Sheldon, who designed the flag. But Leonard notes that he ordered the flag, since Leonard is too disliked in the flag community and gets the price jacked up on him.

To get revenge, Sheldon changes Leonard's wifi password. Leonard retaliates with a shot below the belt: wearing the flag as a toga while he does laundry, going so far as to take his underwear off from underneath the flag while Sheldon watches in horror.

Leonard then rents out his vacant bedroom in Leonard and Penny's apartment to a weird old man named Theodore (Christopher Lloyd). This is the last straw for Leonard, who charges across the hall and bangs on Sheldon's door ("Will you keep it down? What kind of vengeful bed & breakfast do you think I'm running?").

Leonard accuses Sheldon of being cruel and selfish, while Sheldon says Leonard went out of his way to hurt him.

Then Theodore joins the conversation, and observes that their anger at each other is coming from a place of love.

"Seems like with Sheldon moving out, you're in a new phase of your lives and it's easier to fight than face your feelings for one another," he says, showing a surprising amount of insight for a guy who moments earlier was digging for change in Leonard's couch cushions.

They admit that Sheldon moving out has been hard for both of them, and they come to a truce. "I want you to keep the apartment flag, and I'm not just saying that because it touched your genitals," says Sheldon. "And to show that there are no hard feelings, I'm willing to rub my genitals on it as well."

Theodore chimes in with the kind of dirty old man statement you'd expect from him: "Well, if we're rubbing genitals on things, that's where I shine!"

Oh, and Bernadette (Melissa Rauch) went into labor! Next week there's going to be a Big Bang baby!

The Big Bang Theory airs Thursdays at 8/7c on CBS.

(Full disclosure: TVGuide.com is owned by CBS.)