Visser Four was the fourth highest ranked Visser in the Yeerk empire, but after losing the battle for Leera, he was dishonored and came to Earth, where he has taken a human host, an out-of-work actor named John Berryman. Desperate to regain power, he discovered the Time Matrix and used it to rewrite history and make Earth weaker so that the Yeerks could easily conquer it in the future. The Animorphs tracked Visser Four through many historical events, right up to D-Day, where John Berryman got fatally injured. Visser Four abandoned Berryman's body, and Marco killed the Yeerk by throwing it into a burning tank. The Animorphs then rewrote history by visiting the time period where Berryman's parents meet, so as to prevent Berryman's birth and later infestation.