Plane.Raycast

Description

Intersects a ray with the plane.

This function sets enter to the distance along the ray, where it intersects the plane. If the ray is parallel to the plane, function returns false and sets enter to zero. If the ray is pointing in the opposite direction than the plane, function returns false/ and sets enter to the distance along the ray (negative value).

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Plane groundPlane;
    public Transform markerObject;
    void Update() {
        if (Input.GetMouseButtonDown(0)) {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            float rayDistance;
            if (groundPlane.Raycast(ray, out rayDistance))
                markerObject.position = ray.GetPoint(rayDistance);
            
        }
    }
}

See Also: Physics.Raycast.