It's What I Want by Tanya Allan

Don is a bit of a geek. But his intelligence was matched by his kind heart, and his flat mate and friend Steve appreciates his help in his university course work.

Don is a little confused as to why Steve and two other friends want him to join them of a motor cycle tour of France in the summer holidays, but he is pleased to be asked, and goes along. On their first stop, a cool group is playing at a night club, but it is a couples only evening, and Don is persuaded to become Donna for one evening, just so the four friends can see the show.

But no one expected to find Donna still there on the following morning.

In fact, Don never returned. And Donna was anything but a Geek!