Gary Busey has landed a unique role on a unique episode of Two and a Half Men.

The notoriously unstable actor will guest star as himself on the November 14 installment... appearing in a mental facility... alongside Alan... who will be there because he believes he actually is Charlie Harper.

Look for this unraveling to take place after Walden donates the late Charlie's piano to under-privileged kids. That charitable act will force Alan to confront his brother's passing once and for all.

Busey, meanwhile, has played a role on Celebrity Rehab and is 13 years sober.