Horrid Henry's top secret unbreakable code is very simple to understand but very difficult to crack!

 

A = Z
B = Y
C = X
D = W
E = V
F = U
G = T
H = S
I = R
         J = Q
K = P
L = O
M = N
N = M
O = L
P = K
Q = J
R = I
         S = H
T = G
U = F
V = E
W = D
X = C
Y = B
Z = A

 

What does this say? HNVOOB GLZW YILGSVIH PVVK LFG!

[SMELLY TOAD BROTHERS KEEP OUT!]