Cassie looks beautiful in her wedding dress. Sam watches her as she walks down the aisle, the woman he's been crazy about since they first met. Cassie smiles at Sam's family as she approaches the altar. She agreed to a big church wedding purely for the sake of Sam's family, who love a traditional Irish get-together. Sam would have been just as happy having a quiet do in a registry office, but he'd have had to arrange his own murder investigation first because his ma and da would have killed him. There are only a few people on Cassie's side of the church, childhood friends and cops mainly.
He can't stop thinking about the call to Cassie's phone in the middle of the night. He'd just been able to make out Rob's voice, sounding drunk. Cassie had brushed it off as nothing, but in the morning the phone was still lying by her side as if she had fallen asleep holding it, although there was no one on the other end.
She had promised, when he proposed, that it was all over between her and Rob. He knows her well enough to know that she had meant what she said, but ever since that night with the phone call he's been wondering if she regrets that promise. Occasionally he's caught sight of her looking sad, whether because of Rob or the baby or just not being able to go back to Murder, Sam can't tell.
There's a ringing in his ears as Cassie recites her vows, smiling at Sam when she says the old-fashioned "I do" part. He loves this woman so much, he wants her happiness more than anything.
So when it's his turn to say his vows, his voice cracks as he utters those two words:
"I can't."