Alex seemed liked a kind and harmless person, but he was secretly intent on murdering several teenagers who had been responsible for the accidental death of his 10 year old sister six years before in a childhood prank gone wrong. Witnessing his sister's death broke his sanity and he planned revenge.
1974 EditAlex is first seen in the opening scene of the film alongside his older sister Kim and twin sister Robin. As they were walking home from school, they came across an old abandoned convent where inside, a game of hide and seek was being played by 11-year-olds Nick McBride, Wendy Richards, Jude Cunningham and Kelly Lynch. Robin was eager to join the game but her siblings were uninterested. When Kim left to go back to school to get her geography book, she told Alex and Robin to go home and that she would catch up with them later. Alex began to head home but Robin dawdled and eventually ventured into the convent wanting to join the game, causing Alex to leave without her.
A short while later, Alex came back for Robin but found her dead on panes of construction glass; having fallen from a window after being cruelly taunted by the 4 other kids shouting "Kill!, Kill!, Kill!". He witnessed them leave the convent (this was not shown until a flashback at the end of the film) and planned revenge on them when they didn't report the incident to the police and kept it a secret.
For reasons unknown except for the most likely one of personal revenge against the 4 responsible for Robin's death, Alex didn't say anything to the police either. A known rapist was instead blamed for Robin's death and the police spent most of their time during the film trying to capture him whilst remaining oblivious to what really happened surrounding Robin's death.
6 years later, a teenaged Alex and Kim (along with their parents Mr. and Mrs. Hammond) attended Robin's grave on the anniversary of her death. This also happens to fall on the same night as the school prom. During the day, Alex began his plan of revenge for Robin's death by calling Nick, Wendy, Jude and Kelly separately and speaking to them in a creaky voice, he let them know he knew their little secret although Nick never actually received his call as he was running late for school. The other 3 didn't seem to be affected by their calls and dismissed them as pranks.
Also during the day, Alex tore out Nick, Wendy, Jude and Kelly's yearbook photos and left them in their lockers with a glass stabbed on them for each person to find but nobody was able to understand the meaning or seriousness behind this, dismissing them as pranks as with the calls. Alex also got into a fight with the school rebel Lou Farmer in the cafeteria when he was harassing Kim but was let off the hook by the principal (Mr. Hammond) while Lou was suspended indefinitely.
Some time later, Alex snuck into the girl's changing room and broke a mirror whilst Kim and Kelly were inside. When they went outside to try and see if they could find the one responsible for breaking the mirror, Alex secretly took a shard and snuck back out before Kim and Kelly came back.
When the prom started, Alex is first seen helping to manage the DJ system, which was his job for the night. Later on, Kelly and her boyfriend Drew Shinnick snuck off to the school's changing room and began to make out but Kelly refused to continue to full sex, resulting in Drew angrily leaving her in the dark alone. As Kelly was getting dressed, Alex (now wearing all black clothing and a ski mask that was worn earlier by Lou during the fight in the cafeteria) stealthily approached her and murdered her by slitting her throat with the mirror shard he took earlier.
He then ventured outside the school and began spying on Jude and her boyfriend Seymour "Slick" Crane having sex and smoking marijuana in Slick's van. They continued doing so outside but Slick noticed that someone was out there watching them, resulting in him and Jude returning to the van. Whilst they were talking, Alex stealthily approached the van, flung open the back doors and murdered Jude by stabbing her throat with the mirror shard. He then began brawling and struggling with Slick as he attempted to drive away but managed to escape from the van as it tumbled off a cliff (with Slick still inside) and the van exploded into a wreckage.
After this, Alex returned to the school, armed himself with an axe and confronted Wendy in the bathroom resulting in a long chase through the school. Wendy managed to evade Alex several times before hiding in a storage room. Here, she discovered Kelly's body and in a panic, she screamed and ran out of the storage room, only to bump into Alex who then proceeded to hack her to death with the axe.
With only 1 more of the 4 responsible for Robin's death to kill (Nick), Alex snuck backstage where Kim and Nick were preparing to be crowned prom queen and king. Unbeknownst to Alex, Lou and his friends ambushed and tied up Nick moments before as part of a plan Lou and Wendy were planning to pull to embarrass Nick and Kim. Lou took Nick's crown and assumed his position backstage but as the crowning was beginning, Alex (thinking Lou was Nick) approached him from behind and decapitated him with the axe.
Lou's head rolled onto the dance floor, sending the partygoers fleeing in horror. Kim found Nick and freed him but as they were about to escape, Alex confronted them and attacked Nick. Kim tried to intervene but Alex repeatedly pushed her aside. Eventually in the ensuing brawl, Kim managed to grab hold of Alex's axe and struck him on the head with it. Alex immediately stopped attacking, got to his feet and stared at Kim for a moment. At this point Kim realized who the killer was and who she'd just butchered.
Alex then ran outside the school where the partygoers were all standing and where the police had just arrived. Kim and Nick followed him and as guns were raised to shoot Alex, Kim screamed at the officers not to shoot. After collapsing on the ground, Kim and Nick appeared at Alex's side where Kim unmasked him and was deeply shocked to find her brother laying on the ground dying. Alex tearfully told Kim that he saw Nick, Wendy, Jude and Kelly cause Robin's death 6 years ago ("I saw it. I know they did it. They killed her") before dying in Kim's arms whilst crying Robin's name. Kim then sheds a tear for the death of another sibling and mourns her little brother.