Let $ S$ be the set of points in the Cartesian plane that satisfy
\[ \Big|\big|{|x| \minus{} 2}\big| \minus{} 1\Big| \plus{} \Big|\big|{|y| \minus{} 2}\big| \minus{} 1\Big| \equal{} 1.
\]
If a model of $ S$ were built from wire of negligible thickness, then the total length of wire required would be $ a\sqrt {b},$ where $ a$ and $ b$ are positive integers and $ b$ is not divisible by the square of any prime number.  Find $ a \plus{} b.$
1. **Understanding the given equation:**
   \[
   \Big|\big|{|x| - 2}\big| - 1\Big| + \Big|\big|{|y| - 2}\big| - 1\Big| = 1
   \]
   We need to analyze the behavior of the function \( f(a) = \Big|\big||a| - 2\big| - 1\Big| \).

2. **Analyzing \( f(a) \):**
   - For \( 0 \leq a \leq 1 \):
     \[
     f(a) = \Big|\big|a - 2\big| - 1\Big| = \Big|1 - (2 - a)\Big| = \Big|a - 1\Big| = 1 - a
     \]
   - For \( 1 \leq a \leq 3 \):
     \[
     f(a) = \Big|\big|a - 2\big| - 1\Big| = \Big|a - 3\Big|
     \]
   - For \( 3 \leq a \leq 4 \):
     \[
     f(a) = \Big|\big|a - 2\big| - 1\Big| = \Big|a - 3\Big|
     \]
   - For \( a \geq 4 \):
     \[
     f(a) = \Big|\big|a - 2\big| - 1\Big| = a - 3
     \]

3. **Considering the equation \( f(x) + f(y) = 1 \):**
   - We need to find the points \((x, y)\) that satisfy this equation.
   - We analyze the cases for \( x \) and \( y \) in different intervals.

4. **Case 1: \( 3 \leq x \leq 4 \) and \( 3 \leq y \leq 4 \):**
   - Here, \( f(x) = x - 3 \) and \( f(y) = y - 3 \).
   - The equation becomes:
     \[
     (x - 3) + (y - 3) = 1 \implies x + y = 7
     \]
   - This is a line segment from \((3, 4)\) to \((4, 3)\).

5. **Symmetry and reflections:**
   - The function \( f(a) \) is symmetric about \( x = 0 \) and \( y = 0 \).
   - We can reflect the line segment \( x + y = 7 \) in the first quadrant to other quadrants.

6. **Calculating the total length:**
   - Each segment \( x + y = 7 \) has a length of \( \sqrt{2} \).
   - There are 8 such segments (one in each quadrant and their reflections).

7. **Total length of wire:**
   - Total length = \( 8 \times \sqrt{2} = 8\sqrt{2} \).

8. **Finding \( a \) and \( b \):**
   - Here, \( a = 8 \) and \( b = 2 \).
   - Therefore, \( a + b = 8 + 2 = 10 \).

The final answer is \( \boxed{10} \).