Susan Storm-Richards, aka The Invisible Woman, was the only one in the Fantastic Four base. Reed was meeting with other geniuses, Ben was at the game and Johnny out looking for girls. When she started to hear noises in the lab, which she moved to check out. Looking into it, she noticed that a large jar-like tank had been broken open.
Getting a good look at it, she realized what it was, "The Venom Symbiote. Peter had brought it back after AIM had stolen it from Eddie Brock and done some experiments on it. It has to be around here somewhere."
She was deciding on her next move, whether to turn invisible or bring up a forcefield. Or both. However, before that had happened, she felt something smack into the back of her head. She feel forward, bumping her head. As she lost consciousness, she could see a blob overwhelming her.
With the Symbiote...
The Venom Symbiote was having problems. The experiments that had been performed on it had done something to it, both physically and mentally. It's mind was scrambled, confused. It was hard to think, memories mostly gone. Save for memories of Peter Parker, Spider-man. And it's need for Peter, unsure why. It also had the power to replicate itself, making drones, to take more then one host.
It took it's chance to take a new host, the blond female in front of it. Acting quickly to knocking her out. It began to bond and merge with her. It was easy as her mind was muddled and confused by the head blows. As it fused, the symbiote repaired her. But their mental elements were having problems. Looking at the mind it found interesting things. Desires for control, recognition, pleasure and releasing of desires.
Focus on those interesting elements, bonding with it's own focus and desire for Peter Parker. The symbiote eat her clothing, replacing it with a black version of it. Which it turned to look like Latex, which was a hidden fetish of Sue. Also using a more revealing costume, with a cut out spider on the chest to replace the 4 and show off her breasts.
As they merged their minds became more and more in synch, until the damage was repaired. then it was just time before she woke up. But by then, they would be fused.