"As far as Mac knows, that's where she is, and out of the blue, he spots her," Sinise tells TVGuide.com. "He's shocked and surprised by it. She made the choice to end the relationship — it was never something he wanted to do. I think he was a bit stunned by that, and there was never any real closure or resolution. So when he runs into her, there's some unfinished business."
Worse, Peyton's return comes just as Mac has settled into a relationship with emergency room doctor Aubrey Hunter (Madchen Amick). Even though executive producer Pam Veasey tells us Peyton is aware of the other woman in Mac's life, Peyton and Mac can't deny that they still have feelings for each other.
"Peyton's really happy to see [Mac], and it's a fond memory for him," Veasey says. "It was a great relationship, and anything can happen when those feelings are sort of stirred again."
But what if Peyton gets mixed up in a murder? Mac spends much of the episode in his apartment with an injury, setting up a Rear Window-esque plot. Mac sees a briefcase exchange that ultimately leads to a murder, and Peyton is friends with one of the suspects (guest star Gale Harold).
"He can't ignore what he sees, and there are some surprises," Sinise says. "She's caught in the middle of him and the case, and I think she's as surprised by what happens — and what he reveals is happening around her — as he is."
By the end of the season, however, Mac will be in a position to choose between Peyton and Aubrey, which Veasey jokingly likens to a Twilight love triangle. "It will be answered in some form [next season], and we don't know what that is yet," Veasey says. "We like both Peyton and Aubrey, but at this point, all we know is that Mac is in a great place to have a relationship with someone."
Sinise will wait for the answer all summer like the rest of us, but he says he appreciates the opportunity to play something different from the crime of the week. "I like when we have guest stars on the show. They give me an opportunity to work on a level that I don't get to work on often," Sinise says. "Our show is driven by the murder, and that's always going to be the focus because our show is a forensics whodunit. But exploring the characters and what's going on personally is always interesting to all of us."