“What do you think happened, Katherine?” I asked. “Did you eat or drink something bad?”

“I think someone tried to poison me,” she said. “Someone mixed poison with my tea.”

“Or did you take a sip of the potion you made—the one you gave to Mr. Carter? The one you gave to your uncle Cedric?”

“Katherine!” Sissy gasped. “You didn’t. Please say it’s not true.”

“It was only an experiment.” Katherine tried to sit up. “We didn’t expect it to work. We just mixed up a drop of everything deadly that we could find and then we brushed it onto an envelope on Uncle Cedric’s desk.”

“It was Kat’s idea,” Nick said. “We never thought. . . .”

“Uncle Cedric was being so beastly to us,” Kat said. “He wouldn’t give Mummy any money. He didn’t want us in the house. He wouldn’t let us go to school, or let Sissy’s legs get better. Mummy kept crying.”

“We never wanted to kill anyone,” Nick said.

“What about Mr. Carter? You didn’t try to kill him?”

“No. I swear it wasn’t us,” Nick said. “We didn’t go anywhere near him.”

“Then he must have made the ultimate sacrifice for you,” I said. “He realized what you had done and he drank your potion.”

“I know.” Kat was crying now. “I don’t want him to die.”

“He’s gone off in an ambulance. They may be able to save him,” I said. “But it will mean that you two have to tell the truth to the police.”

“Will we go to prison?” Nick asked in a quavering little voice. “Will they hang us?”

“I’m sure they’ll take your age into account,” I said, “and the fact that you didn’t really intend to kill anybody.”

“What’s this?” Irene stood in the doorway with a glass of barley water in her hands.

“We made a potion and put it on Uncle Cedric’s envelope, Mummy. We thought he’d lick it and feel ill. We didn’t think it would kill anybody. And then when Uncle Cedric died, we were so frightened. We decided to keep quiet about it and they’d never think it was us. But Georgie worked it out . . . she’s really clever.”

Irene put down the glass, shut the door behind her and came into the room. “Who else have you told about this?”

“Nobody,” I said. And as I said it, a cold shiver went down my spine. I remembered what she had said, that time in the Long Gallery: she was the mother lion, ready to fight to protect her cubs.

“Where is this potion?” Irene asked calmly.

“In the schoolroom. It’s labeled copper sulfate. At the back of the shelf.”

“Go and get it, Nicholas.”

“But Mummy . . .”

“Do it.”

She was cold and frightening, and she was standing between me and the door.