-Draft 1, Written without any coffee-
There are aliens among us. They pass by Earth, using this little corner of the galaxy as a way station on their travels. Some drop down to the planet's surface to replenish stores of oxygen and water, others to perform more mischievous deeds.
The problem is, humans are starting to catch on. Governments are tracking those faint traces on their radar, sending out hunter teams to retrieve any visitors and examine them and their technology, regardless of the alien's intent.
Aliens aren't helpless though. They often manage to slither out of a tight spot (sometimes literally!), and there's even the occasional escape from Area 51. Take last week's escapee for example. A tall and slender silver skinned creature just disappeared from the holding cells. Nobody seems to know how it happened, and heads will roll for the lack of security!
In fact, the desk jockeys are so incensed about the whole affair, that they've completely failed to realize that on the day of the escape, there was one more soldier on the base than their should have been. She was a voluptuous blond who filled out her uniform perfectly, showing enough cleavage to keep anyone from paying too much attention to her credentials or where she was dead.
Who could have guessed that they'd picked up a shapeshifter after all? And certainly nobody could have foreseen that she'd have wriggled through an air vent to the laundry room and shaped herself to fit into the first uniform she found laying around! Fitted herself so perfectly in fact that when a lone veteran guard finally caught on to her, she managed to charm him into helping her escape.
With her ship damaged and confiscated, the alien is stuck with just a former captor to keep her hidden from the prying eyes of Big Brother. And to make things worse, the oxygen rich atmosphere has sent her body into overdrive! She goes into heat regularly, needing to mate or else risk permanent damage to her body and loss of her shape shifting abilities. Her human savior will have to do, close enough in physical design to stave off her moments of lurid, intense heat.