Based on a true story, Ken Loach’s powerful and disturbing British drama about a single working-class mother with four children from four different fathers is made unforgettable both by stand-up comedian Crissy Rock’s lead performance and by the filmmakers’ determination to make the story as messy and as complex as life itself. After many abusive relationships, Maggie, the heroine, settles down with a gentle Paraguayan refugee (beautifully played by Vladimir Vega), but then has to contend repeatedly with the state taking away her children. This sounds like a simple antiwelfare polemic, but Loach doesn’t allow us to walk away from the movie with any settled or monolithic message. As written by Rona Munro and played by Rock, Maggie is a volcanic conundrum, and the deeper we become involved in her fate, the less sure we become about anything. Highly recommended. Music Box, Friday through Thursday, January 20 through 26.