Daniel never got the chance to celebrate his last 5 birthdays. 5 years ago he mistakenly got a chance have a wish granted by a genie. He wished for “a great birthday”. That wish held up for every birthday of his. The effect is that every year, on his anniversary, he transforms into Sarah.
Sarah is a great girl. On the first transformation, she sweet-talked the security from a hotel to let her party in the pool. She enjoyed filming some of it for Daniel.