During the SETI program, many of Earth's best scientists grouped together and sent out transmissions for possible alien listeners. Information about Earth, it's inhabitants, DNA, progress, etc - all given out in hopes that someone was listening.
What is being kept quiet is that we actually got a response. The first was instructions on how to make an endless fuel source with little effort so we assumed they had good intentions. The second was instructions on how to make a strand of their DNA and splice it with our own.
A government team led by a Dr. Xavier Fitch went forward with the genetic splicing, with the added touch of making the alien/human hybrid female believing she'd be easier to control then a male. Named Sil, she would prove them so wrong when she escaped and the government only barely managed to stop her before she successfully mated with a human male and began the process of wiping out life on Earth.
The threat of Sil was stopped, but that doesn't mean the research stopped. Someone would try again, someone who in their determination to not make the same mistakes would make whole new ones.
Another group has made what they think is a more streamlined and controllable version of the hybrid. The only downside was that their process couldn't control the hybrid's gender and as a result their hybrid is a male named Kal. Oh, they did their best to contain him - even going so far as to make sure the staff and scientists directly studying him were all males. They intended to let him live out his whole life cycle like that, harmlessly tucked away for study.
But a moment of sloppy controls led to him hearing his first female voice, and something has awoken in Kal....
A tribute to a horrible movie series that's dear to my heart.
Basic Premise - A male human/alien hybrid escapes from a government lab on a mission to mate with and impregnate as many human women as he can. How hard the content is can be debatable with sci-fi handwaving.
Of course there will be organizations and individuals that try to stop him. Fighting them off will be important to the plot too. One has to survive if one is going to mate.