The Graingers' apartment was trashed. After Beckett and Castle watched the paramedics load Mrs. Grainger into an ambulance and drive away, they went upstairs to observe the damage. Ryan and Esposito were kneeling in front of the kitchen sink, where shards of the broken window over the sink were scattered. There were smears of Mrs. Grainger's blood on the wall behind them and broken dishes carpeting the floor.

"Hank Grainger said he was in the bathroom washing his hands when he heard the window break," said Esposito, straightening up and facing Beckett. "He ran in here and saw someone all in black wearing a ski mask hitting his wife with a wrench." He pointed to a rusty monkey wrench lying on the kitchen floor amongst the shattered dishes and window glass.

"Assailant probably wore gloves, but we should test it for prints anyway," said Beckett, surveying the rest of the room. "What happened after he walked in and saw the attack?"

"Attacker jumped back out the window," said Ryan, also standing up. "Probably ran down the fire escape. Grainger said he wanted to chase the guy but he was more concerned with his unconscious wife."

"The person who attacked her was wearing all black?" said Castle, obviously disturbed. "Like a ninja?"

"Not a ninja," said Beckett. "There might not have even been an intruder."

"You think her husband did this?" asked Ryan. The thought had been on his and Esposito's minds, but because his alibi had already checked for Andrea's murder, they'd dismissed it. Sure, an attack from a mysterious masked intruder was unlikely, but not impossible.

"He was cheating on her," said Beckett. "So what if she found out, they argued, and it eventually turned to violence?" She was doing what she always did when solving a case, spinning a story. It amused the guys that she always chastised Castle for making up stories about how someone was murdered, not realizing that she did it all the time. Her stories just happened to have more hard evidence and less organ-stealing prostitutes than Castle's.

"Yeah, but where'd he get the wrench?" pointed out Esposito.

"Maybe he was in the middle of trying to fix a leaky faucet," she said, eyeing the gleaming spigot over the sink. "Canvass the area anyway, on the off chance that our attacker ripped off his mask as soon as he was clear of the area." With twin nods, Ryan and Esposito left the apartment.

"But if it was the husband, how do we find out for sure?" asked Castle.

"We interrogate him," she said, resisting the strange urge to add "Duh!" "Honestly, I know your head is a little messed up right now, but you know that much."

"Yeah," he said sarcastically, "we ask him if he attacked his wife and he'll just give us the straight answer. He's already lying." Beckett frowned, recognizing the truth in his words. "We have to lawyer him!"

"What?"

"We ask him a bunch of questions and try to find a flaw in his story," he explained. "Like Encyclopedia Brown." She rolled her eyes, but nevertheless agreed with his idea.

"He's at the hospital with his wife," said Beckett.

"Maybe we could stop at the psychiatric ward on our way to talk to him," said Castle, following her out the door. "I could say hi to all my old friends. Max Hopper, George Drew, Mackenzie Green- of course, they were all the same person…"

Beckett stepped into Cindy Grainger's hospital room just as the injured woman's eyelids were beginning to flicker. Mr. Grainger, who was sitting beside her cot and clutching her hand, turned around to glare at Beckett as Castle walked up behind her. "She's waking up," he hissed, his gaze returning to his wife's face.

He seemed extremely worried about his wife's well-being, which caused Beckett to retract her theory a centimeter. Mrs. Grainger opened her eyes, and they rotated to focus on her husband. "Hank," she breathed, her grasp on his hand tightening.

"I'm here," he said softly, leaning over her. Beckett felt Castle shift nervously behind her, apparently feeling awkward about intruding on the scene in front of them.

"So brave," said Cindy admirably. "Chased him off, saved my life." Kate frowned. Unless Cindy Grainger's brain was seriously scrambled, hers and Castle's "Hank Grainger Attacking His Wife" story was looking pretty far from the truth. "Thank-you."

Hank gulped, his eyes downcast with guilt. "Don't thank me," he said, sullen. "You could've died, and the whole time I was… I was an awful husband." She frowned, looking confused. "Cindy… I cheated. I hate myself for it, but I did."

"I know," she sighed. "I heard you talking to her on the phone the other night."

"The other night?" said Beckett, interrupting. "But you said you ended things with Andrea months ago."

Cindy seemed surprised to see Beckett and Castle in her hospital room, but continued to speak anyway. "Andrea?" she said. "No, it was Kristen he was talking to. That girl you were visiting up in Scarsdale."

At that moment, two things ran through Castle's mind- a memory of a woman saying "Men think they're smart. The trick is to keep letting them think it" (he wasn't quite sure where the memory came from), and the image of Andrea's sister, Kristen, who lived in Scarsdale.

Kristen Methay sat at the table in the interrogation room, looking incredibly stressed out and worried. Castle and Beckett watched her from the observation booth. They'd picked her up just over an hour ago, with a warrant to search her apartment for arsenic and the black clothing of Mrs. Grainger's assailant. "You mind if I'm in there alone?" said Beckett.

"Are you actually asking?" he replied, slightly amused.

"No." She walked away from him and into the interrogation room. Kristen glanced up as she sat down across the table, but gave no other indication that anyone else had entered the room. "Kristen," said Beckett.

"I didn't do it." It sounded like she'd repeated the phrase over and over again for the past hour, rehearsing for a badly delivered lie. Or maybe she was trying to convince herself, to erase from her mind what she had done.

"You loved Hank Grainger, didn't you?" said Beckett. She tried to stay calm, almost comforting. Despite the 99% surety she had that Kristen had poisoned her sister, she knew that the girl wasn't a cold-hearted assassin.

"Does it matter?" she asked, looking away. "He'd never want me now." Kate took that as a semi-confession and barreled ahead.

"You were in love with him, but he'd never leave his wife," she continued. "And when you found out that he had another mistress, that she was your sister, you must have been furious. So you came up with a plan. You could get poison somewhere. It wouldn't be too hard to make a copy of Hank's key." She paused, waiting for Kristen to object or to ask for a lawyer, but Kristen remained silent. "It must have been tearing you up inside, and you still didn't have the man you killer her for. The job wasn't done. So you broke into the Graingers' home and attempted to kill his wife, having no idea that her husband was in the other room." Kristen stared at her hands folded in her lap, and Kate had a bizarre moment where she felt like an elementary school teacher reprimanding a child for biting another kid, instead of a homicide detective accusing a woman of siblicide.

"Andrea had no idea how to take care of Henry," said Kristen. "She was lost, and she barely even loved the kid. He was a curse to her. I loved him, and I loved Hank, more than anyone else did." Kate nodded slowly, extracting a confession form from the file in front of her. "We could have been a family. A happy little family and it would have been perfect." A sob she must not have been expecting erupted from her throat, and she sniffed.

"Did you poison Andrea Methay?" Kristen looked like she just wanted to get away from Beckett, away from the sharp lights of the interrogation chamber.

"Yes," she whispered.