The expression, "if it's the last thing I do," means that a person has an extreme determination to do something. In highly dangerous predicaments, such as the ones the Baudelaires found themselves in, it can become literal. Violet had determined to stop Olaf from harpooning Dewey Denouement if it was the last thing she did...
"I'm going to make Olaf pay if it's the last thing I do," Klaus said, clenching his jaw.
"Not you too," said Sunny anxiously.
"I don't mean it literally, Sunny," Klaus reassured her. "I mean we have to use our wits to figure out Olaf's plans and do our best to thwart them."
"Trial?" asked Sunny.
"I'm not counting on the trial," said Klaus. "The other justices are probably just as useless as Strauss. Olaf didn't seem worried."
"Laughed," Sunny said.
"We have to think about everything we heard as flaneurs today and set our minds on defeating Olaf. It's what Violet would want us to do," said Klaus.
"Cocktail. Valuables," said Sunny, thinking of the two teachers and the Vice Principal discussing their party invitations.
"Yes, Olaf's planning robbery at his cocktail party but it sounded like he's going to hold it in the lobby. He can't until after the trial. I'm thinking of things we might do before that."
"Sugar bowl," said Sunny.
"Yes. Olaf has the phrases now. That despicable coward Dewey! If he was going to cave in, why didn't he do it before Violet was shot?"
"Failed us too," said Sunny, blinking back tears.
"If we manage to get away during the trial, I'll try to get to the Vernacularly Fastened Door and type in the wrong phrases to jam the lock," said Klaus. "That's Plan A."
"Plan A?" asked Sunny.
"It's our private code," said Klaus, "When I say 'Plan A' then you'll know I'm going for the laundry room door."
"With," said Sunny, meaning "I want to go with you, Klaus."
"We may need to split up," said Klaus. "There's still one more thing; we have to find the Medusoid Mycelium and get rid of it before Olaf can use it."
"Where?" asked Sunny.
"I don't know. We have to think," said Klaus.
"Room?" said Sunny.
"It would be too risky to have it in his room," Klaus said. "What if spores got loose while he was asleep?"
"Esmé. Guarding," said Sunny.
"You're right, Sunny. Esmé said she was guarding something," Klaus said. "But Olaf said she was up in the tanning salon all day... The mushrooms are probably up there somewhere."
Klaus got out his commonplace book and held it close to the crack at the bottom of the door where light was coming in. He read through all the notes he had taken about what Violet had observed on the roof.
"The boat Carmelita was riding had a figurehead of a diver being attacked by an octopus... That's it! The safest way to handle the fungus would be to keep it trapped inside the diving helmet until he was ready to use it."
"Eureka," said Sunny, which meant, "We've solved the mushroom mystery."
"That will be our Plan B," said Klaus. "One of us needs to get to the roof and throw the helmet into the sea before Olaf can use it."
"Unsafe," said Sunny nervously.
"It's a risk we have to take," said Klaus. "It will save lives. Better yet, it will foil Olaf. That's what I want more than anything, to avenge Violet. What else can I do now?"
"Last thing I do," said Sunny in agreement.
"That's about all the plans I can think of," said Klaus. "We should try to get some sleep if we can."
"Can't" said Sunny.
"Neither can I," said Klaus. "I was hoping you could."
Both sat leaning against each other, overwhelmed by grief. It took hours before they dropped into exhausted slumber.