Halflife by Mark Michalowski

To lose your memory once may be regarded as a misfortune; to lose it twice looks like carelessness. The Doctor's not himself. He's not quite sure who he is, but he's definitely not himself. It doesn't help that he's forgotten quite why he came to the colony world of Espero in the first place, but he's sure it was something important. Whatever the reason, he's not the planet's only visitor. Before long, he's engaged in the search for a time-bomb - a time-bomb that could have consequences not only for Espero, but for the Doctor himself - and his missing past.