Marwood (Paul McGann) and Withnail (Richard E. Grant), two unemployed actors living in 1960s London, feel the need to get away from their decaying hovel of a flat for a weekend in the country, and so borrow the Lake District cottage of Withnail's eccentric and homosexual Uncle Monty (Richard Griffiths).
However, when they get there, the cottage is in terrible condition, they don't have any food, and the local poacher wants to kill them. After being threatened with a dead fish, massacring a chicken, and shooting a hole in the ceiling, Monty appears in the dead of night. Rescuing them from a death that almost certainly would have come about, his presence soon becomes unwanted after he takes an obvious liking to Marwood. Marwood narrowly avoids "having a buggering" as Monty attempts to assault him one night, and when morning comes it seems Monty has fled in disgrace.
Marwood and Withnail return home, finding drug-addled Danny (Ralph Brown) and a notice of eviction waiting for them. Marwood then receives word that he has got the leading role in a play, meaning he will be leaving for Manchester. The day he departs, the pair walk through Regents Park, Withnail drowning his sorrows in a stolen bottle of Monty's wine as he gives a rare display of emotion, realising he is about to lose the only person he cares about. Marwood heads to the station alone, as Withnail delivers the performance of a lifetime, leaning on the railings of the wolves' enclosure in the rain, and regaling them with a dramatic speech from Shakespeare's "Hamlet". The credits roll as he walks back across the park, alone.