A man has a watch that makes him invisible when pressed. However he can only remain invisible for 15 minutes or else he will stay invisible forever.

Sam Casey is caught in an explosion which makes him invisible. By using a stabilizer in the form of a wrist watch, he can remain visible. By turning off this stabilizer, he can become invisible again for short periods of time. Any more than fifteen minutes of invisibility will make him disappear entirely. He uses this invisibility as a secret agent.—Anonymous