Danny is an engineer at the watchtower, and it is everything he ever dreamts of
sure he's not on the moon, but he's in space! he meets and talks to aliens on a daily basis! he can just go out of the station, fly around and come back in! he is very happy with his job.
so when danny went back in the watchtower after fixing a panel outside, finding out that everyone was being mindcontrolled, and danny is the only one who was not affected because he was outside-
well, the only thing he could really do is put the watchtower on a lockdown, and hope someone from the outside notices something before it's too late.(no matter how slim the chances of that are, afterall, almost everyone in the justice league was present in the tower, some kind of urgent meeting)
they don't want a mindcontrolled superheros flying around afterall.
or: extremely depuffed engineer danny fenton is stuck alone in the watchtower with mindcontrolled superheros, he needs to save the heroes before it's too late, with only his invisiblity, minor ice powers, flight, and night vision in his arsenal, anything else would be too draining and risky, he doesn't have any ectoplasm here to support him afterall.
the first person danny tries to break free from the mind control is the red robin
"why me? why not batman, or superman?"
"you're the only one who I realisticly could pin down long enough to break the mind control..."
"oh uh-and of course because you're the smartest-"
danny has acquired an ally!