The Wide Window picks up where The Reptile Room left off. The Baudelaire children, Violet, Klaus and Sunny have just left their Uncle Monty's house and are on the way to meet their new guardian, Aunt Josephine. As with Uncle Monty, she is not really a close relation and in fact, not even their aunt, but she has agreed to take them in.

Aunt Josephine is a person crippled by her fears, which are numerous. Everything from doorknobs to realtors terrifies the poor woman. Her biggest fear is Lake Lachrymose, where her husband Ike met his unfortunate demise. Lachrymose is another word for sorrowful or tearful and the name is well used in this book. Her only remaining love is grammar and she continually corrects the children. Her library is filled with books on grammar and has a very large window that overlooks Lake Lachrymose.

After the children arrive at their Aunt Josephine's home, they meet someone claiming to be Captain Sham. However, the children discover his true identity immediately. It is Count Olaf, who has managed to track them down once again.

Although the children are quick to catch on to his disguise, Aunt Josephine is not. She is taken in by his charms and seems almost smitten by him. Too quickly, she realizes just how wrong she is about this man. The very evening after meeting him, the children find a suicide note left by Aunt Josephine and the wide window in her library has been smashed, indicating that Aunt Josephine jumped to her death.

The note contains the harrowing message that the Baudelaires are to be left in the care of Captain Sham. It's up to Violet, Klaus and Sunny to try to stop Count Olaf's plan before they are delivered into his clutches once more.

By working together, the children discover that their Aunt Josephine faked her death and that she left them a hidden message within the suicide note. They rush to figure out the message as a storm approaches Lake Lachrymose. The storm hits just as they finish compiling the information they need and they barely escape as Aunt Josephine's house tumbles into the lake below.

The Baudelaires rush to find their Aunt Josephine to prove that they are not to be left in the care of Count Olaf, but the storm may outwit them. It will be up to the Baudelaire children to combat the forces of nature, the fears of an adult and the wiles of Count Olaf if they are to escape his nefarious plot this time.