According to Wikipedia, the massacre was the result of an attempted false flag operation by British-aligned paramilitaries in Northern Ireland.
The Miami Showband killings (also called the Miami Showband Massacre) was an attack on 31 July 1975 by the Ulster Volunteer Force (UVF), a loyalist paramilitary group. It took place on the A1 road at Buskhill in County Down, Northern Ireland. Five people were killed, including three members of The Miami Showband, who were one of Ireland’s most popular cabaret bands.
The band was travelling home to Dublin late at night after a performance in Banbridge. Halfway to Newry, their minibus was stopped at what appeared to be a military checkpoint where gunmen in British Army uniforms ordered them to line up by the roadside. At least four of the gunmen were soldiers from the British Army’s Ulster Defence Regiment (UDR), and all were members of the UVF. Two of the gunmen, both soldiers, died when the time bomb they were hiding on the minibus exploded. The other gunmen then opened fire on the dazed band members, killing three and wounding two. It is believed that the bomb was meant to explode en route, so that the victim band members would appear to be IRA bomb-smugglers and stricter security measures would be established at the border.
The documentary apparently explores allegations that a British intelligence officer planned and/or led the operation.