Three 12-year-old kids discover a mummy in the basement of a dead man's house. It comes alive due to the conjunction of the moonlight during that time of the month. They are scared of him at first, but with time discover he is friendly, if clumsy and confused.
The kids name the mummy Harold, and decide he will temporarily take up residence in one kid's bedroom. After paying a visit to their Halloween-obsessed friend, they discover that if the mummy is not put back in his coffin before midnight on All Hallow's Eve, the mummy will cease to exist.
Luckily, the sarcophagus is in the Egyptian exhibit at the local museum, but there are a few obstacles. For one thing, Harold's unusual appearance may attract unwanted attention as Halloween night draws closer. Also, someone powerful, and evil is out to get Harold, meaning the kids to save him before midnight on Halloween