On this page of our guide to Destroy All Humans you can find useful tips on how to complete the Mr. Crypto goes to capitol city mission. You'll find out how to get to the Octagon.
Taking over the body of a scientist
After you set off outside the White House, you will notice a scientist you need to follow. Follow him by using a body of any pedestrian, so that no one may discover who you really are.
When you approach the bridge, take over his appearance and then head to the object.
Crossing the bridge
There are many soldiers on the bridge. You can't get exposed or identified by them. If you do that, the mission will fail. You will have to start all over from the beginning of the sequence.
Constantly use your camouflage. Take over the thoughts (Cortex Scan) of all men around you. It will renew the power of your disguise.
There's a challenge at the end of the bridge. You need to quickly disarm the sensor behind the railings and then use Forget on the men around you. Run to the safe zone when everybody around you are confused.
Reaching the building
You still have to be in the body of the scientist. If you die at this point, the game can load a checkpoint without disguise. In other words, this will block the mission. You'll have to reset it and start all over again.
As a scientist, approach the soldiers guarding the entrance. The password is Blue Rider. After entering the protected area, you can freely change form and reach your ship.