Simplest transparent surface shader

Unity Engine
1

I’m trying to learn more about surface shaders. Right now I’m trying to mimic the behavior of the standard transparent shader, but I can’t get the transparency to work as desired. Even when the alpha is set to zero, there’s still a strange gray being rendered.

Shader "Custom/NewShader2" {
    Properties {
        _Color ("Color", Color) = (1,1,1,1)
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
        _Glossiness ("Smoothness", Range(0,1)) = 0.5
        _Metallic ("Metallic", Range(0,1)) = 0.0
    }
    SubShader {
        Tags {"Queue"="Transparent" "RenderType"="Transparent"}
        LOD 200
       
        CGPROGRAM
        // Physically based Standard lighting model, and enable shadows on all light types
        #pragma surface surf Standard alpha

        // Use shader model 3.0 target, to get nicer looking lighting
        #pragma target 3.0

        sampler2D _MainTex;

        struct Input {
            float2 uv_MainTex;
        };

        half _Glossiness;
        half _Metallic;
        fixed4 _Color;

        void surf (Input IN, inout SurfaceOutputStandard o) {
            // Albedo comes from a texture tinted by color
            fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
            o.Albedo = c.rgb;
            // Metallic and smoothness come from slider variables
            o.Metallic = _Metallic;
            o.Smoothness = _Glossiness;
            o.Alpha = c.a;
        }
        ENDCG
    }
    FallBack "Diffuse"
}

What exactly is required to get it to behave as the default standard transparent shader?

2

Why mimic it if you can just look at the source:
http://forum.unity3d.com/threads/builtin-unity-shaders-source.2085/page-2