ALL WRONG on nuketown zombies if you knife a certain nuke shelter you can hear marlton trapped inside trying to get out. The other scientists probably locked him in as a joke and got killed by the missiles that you shoot at earth on MOON. luckily marlton was locked inside and survived the missiles. afterwords he probably escaped wondered what happened saw zombies ran found the bus and jumped in . eventually somehow meeting the others. all facts found within the games he mentions being betrayed by his friends. and on a easter egg on nuketown zombies you can hear him trapped in a bunker cursing and trying to get out.