Solution: First, compute $ g(3 + 4i) = 3 + 4i $, since $ g(z) $ returns $ z $ itself. Then, $ f(g(3 + 4i)) = f(3 + 4i) = (3 + 4i)^2 = 9 + 24i + 16i^2 = 9 + 24i - 16 = -7 + 24i $. The result is $\boxed{-7 + 24i}$.