plotting – How to plot a line from a surface

I have a surface difference that I have defined as an expression of x and t and can plot it, but I’m running into some trouble when I try to plot a 2d slice of the plot by itself on the line x = 0.25. I have tried to plot the 2d surface by defining a new function diff2d where I use With to set the value of x to 0.25, but this is not giving me any output for the plot.

What is a way to plot this line that I’m missing?

neq = 40;
sea(n_) := 2*Integrate(-Sin(n*Pi*x), {x, 0, 1})
vn = Table(0, {n, 1, neq});
Do({vn((n)) = sea(n) Sin(n*Pi*x) Exp(-(n*Pi)^2 * t)}, {n, 1, neq})
vn;
v = Total(vn);
u = v + 1;
difference = u - Erfc(x/Sqrt(4 t));
diff2d = With({x = 0.25}, u - Erfc(x/Sqrt(4 t)));
Plot3D(u, {x, 0, 1}, {t, 0, 1}, AxesLabel -> {"x", "t", "u"})
Plot3D(Erfc(x/Sqrt(4 t)), {x, 0, 1}, {t, 0, 1}, 
 AxesLabel -> {"x", "t", "Erfc"})
Plot3D(Evaluate(difference), {x, 0, 0.5}, {t, 0, 1}, 
 AxesLabel -> {"x", "t", "Difference"})
Plot(Evaluate(diff2d), {t, 0, 1}, 
 AxesLabel -> {"t", "Difference on x = 0.25"})