Percy, Annabeth, and Grover accept a favor from the goddess Hecate: they must take care of her magical mansion and creatures during Halloween in exchange for a college recommendation letter. What seems like an easy job quickly turns into chaos when Grover accidentally causes magical problems and the creatures escape.
The trio must travel through dangerous mythological situations to recover everything and repair the damage before Hecate returns. Along the way, they learn about responsibility, teamwork, and trust. In the end, they succeed, fix the situation, and Percy earns his recommendation letter.