As you may have suspected, you are now GLaDoS' new test subject for the Apeture Science Enrichment Center. You have the portal gun, the Advanced Knee enhancements integrated into your legs and a backpack full of frag grenades. Keep in mind that like Chell, you start out with absolutely no idea what's going on or what GLaDoS true objectives other than completing a series of 'tests', followed by the promise of delicious cake. You progress through the same stages as Chell, albeit at your own discretion.
So, what kind of experience will you have in your newfound playground?