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The book begins with Alyssa Redwood celebrating the Jewish Sabbath with her family and her family's friends the Hawthorns, their oldest daughter Shana being Alyssa's closest friend. During their meal, it was mentioned that both of their younger siblings would attend an elementary school field trip the following morning. During the bus ride to the field trip however, there was a crash that killed most of the passengers. During the hospital visit, it was reported that Alyssa's brother Adam was the only one of the immediate survivors (not including the driver) that survives, a depressing blow to the Hawthorns.
While resting near her recuperating brother in the hospital, Alyssa heard static and came to, realizing she couldn't move, but out of her peripherals she spied a shadowy figure looming over her brother. After a brief panic, Alyssa was roused by her parents and learned that she was having a nightmare. A few days later Alyssa attended the funeral for Denise Hawthorn (Shana's sister who died in the crash). During the funeral Alyssa noticed that a couple of kids had become ill and had nosebleeds. She also suspected that she saw the shadowy figure once more.
At school, Alyssa sensed a static presence that practically lulled her into a half-doze where she had an up-close vision of the apparition, which caused her to jump in surprise. She realized that it was focused on another classmate who apparently saw it as well. The student violently confronted her about it later, claiming that he saw his deceased brother asking for his help and finally declared that he was off to help his dead brother. During a visit to her friend Shana, who was mourning, Alyssa learned that Shana had similar visions and had sunk into depression.
Upon returning to school, Alyssa was approached by Deputy Yew about the disappearances of multiple students, including the one that confronted Alyssa previously, and upon leaving the school, Alyssa encountered a strange tree that she never recalled seeing before on her running trail. On her second visit to the Hawthorn's, Alyssa worried as Shana drifted into a catatonic state from her hallucinations and was appalled when her parents were hesitant to seek help due to mourning traditions. Alyssa's fears were justified when Shana disappeared overnight.
Knowing that the shadowy apparition was to blame, Alyssa watched as it became more aggressive and continued to fully elude the police force, who partially suspected the bus driver who cashed the bus and then vanished was to blame. Alyssa's grandmother revealed that she had some knowledge of what had been going on and explained to Alyssa that the odd tree was probably the creature's totem from the "shadow world" to ours. Later, Alyssa had a vivid nightmare where she was awakened by an eerie version of Shana asking for her help. Alyssa followed her and realized that it was a trap, just barely managing to escape and wake up, but when she did she realized that she woke up in the same spot her nightmare left off, meaning she had been physically transported in her nightmare.
Alyssa conjured a plan to rescue the real Shana from the creature's grasp by attacking its totem, and the plan got her trapped in the shadow world, but through cooperating with Shana in the shadow world they managed to escape, although Shana didn't survive. Alyssa's family tried to help her with psychiatric help, but the Slender Man only grew more aggressive and ended up kidnapping Alyssa's brother while at the same time stalking her at every turn. Eventually, the bus driver reappeared and was arrested, but with a message for Alyssa. When he spoke to her, he explained that the Slender Man would hunt her down until he had her, but hinted at a way that Alyssa might have been able to save her brother.
Thinking she might at least be able to exchange her life for her brother's, Alyssa returned to the Slender Man's tree and sang herself to sleep, allowing herself to be sucked into the Shadow World, where the Slender Man had prepared a game involving Alyssa's captured friends and classmates that proved highly dangerous with any sort of physical contact. After doing her best to elude them, Alyssa followed the shadowy variant of her brother to the Slender Man's final form where she managed to wake up the real Adam, although he was still as dangerous to her as the others were. She just barely managed to pull her brother away from him and run.During the pursuit by her shadow-world peers she ended up injuring herself but woke up as a result with Adam alive and still with her. Thinking she'd escaped for the time being, Alyssa tried to get herself and her brother some help, but the Slender Man was waiting and captured her, leaving her brother (although he had no recollection of the most recent events).
Dexter Morgenstern's version of the Slender Man derives from the phenomenon of the Slender Man being what's called a "shadow person," seemingly incorporeal beings that are speculated to originate from a parallel "shadow dimension." Originally appearing in nightmares, variants of the Slender Man/Shadow People are commonly associated with fits of sleep paralysis and hallucinations, often involving feelings of static, numbness, and the inability to speak.