In "Rugrats Go Wild," Nigel Thornberry hits his head, which makes him act like a young child.
The 2003 animated film crosses over Nickelodeon's "Rugrats" and "The Wild Thornberrys." Nigel is normally an eccentric, experienced wildlife documentarian, but an accident leaves him behaving like a toddler. The joke works because his sudden childlike personality contrasts sharply with his usual adventurous confidence.
Nigel's head injury is part of the film's broad slapstick comedy rather than a realistic medical storyline. The movie also uses the situation to bring him closer to the Rugrats, who are accustomed to dealing with unusual behavior and quickly accept him as one of their own.
A common mix-up is to describe the event as amnesia or to say that Nigel simply becomes a baby. Those descriptions capture the result, but the specific event asked about is that he hits his head.