It is set in Garston, an economically depressed area of Liverpool and follows the dramatic and eccentric happenings within one family on the day of its youngest daughter's first Holy Communion.

[1] The Potts family includes father Joe (Andrew Schofield), mother Sally (Lisa Parry), teenagers Paul (Dave Hart) and Paula (Lauren Steele), young children Olivia (Jasmine Mubery) and Karl (Adam Bailey), and the Potts's part-time lodger, Magic (Lenny Wood).

As Joe gets drunk and his attempts to enliven the social atmosphere with jokes and banter fall flat, the cracks in the family's relationships are revealed: Sally wonders whether she is still beautiful or appreciated; Paula, who throughout the film talks out loud to and continually confers with her imaginary friend Georgina, worries over the news that her beloved boyfriend DJ Worm (Mick Colligan) is planning to move to Ibiza without telling her; Magic tries to console her while suppressing his obvious attraction to Paula.

At the social club, Joe and Sally find their relationship strained to the breaking point by both their own failure to communicate and by the charms of local gangster One Dig (James McMartin), who dated Sally years ago and is still willing to offer her a wedding ring.

National press coverage of Under the Mud was limited, due to the lack of a theatrical release, but mostly positive.