When he's interrogated by unknown figures, Alex Mason has to remember the location of a broadcast station, to stop a horrifying biotic weapon attack. During the progress, he begins to suspect the true identity of his Soviet friend-Victor Reznov.
Treyarch actually planned to bring the Radiant mod tools back for the original Black Ops, but the plan got scrapped partway through - only a partial set of tools ever saw the light of day, which is why Black Ops never had a proper custom map scene.
During the mission Project Nova, Reznov mentions in passing that his own father was present at the Battle of Stalingrad, the same fight Reznov himself survived back in World at War. It's a blink-and-you'll-miss-it line, but it adds a whole extra generation to a character whose backstory already carries most of the game's emotional weight.
Steve Blum voices a 'Sgt. Mason' in the console spin-off Final Fronts, two years before Black Ops made Alex Mason a household name in the series. Completely different character, just a funny bit of naming overlap nobody really talks about.
That faded 'Welcome to Nuketown' billboard isn't just set dressing. The population number on it genuinely changes to match how many players are currently in your match. It's such a tiny detail most people never look up long enough to notice, but Treyarch carried the idea forward, giving it an actual gameplay job as a kill counter in the Black Ops II zombies version of the map.
