In 2009, Elton John calls his early-1970s mentor and tour mate, Leon Russell and invites him to make a joint album. Early in 2010, the camera is in the studio as John coaxes a nearly-burned-out Russell, recovering from brain surgery, into composing, playing, and singing. Neil Young, Brian Wilson, and Booker T. Jones come in for a day or two, recording tracks. T Bone Burnett produces. Archival footage shows the young rockers, Crowe interviews his stars, and there’s a sound montage of hits on which Russell was a studio musician. By October, 2010, the CD “The Union” is ready for release; there’s a concert in Boston. Friendship gets an encore.