Language alert on these two clips from the documentary.
The first is the trailer.
The second is an excerpt.
We know about Clapton, who came to Cream from John Mayall's Bluesbreakers. What's lost to history, in the US, less so in Britain, is where Baker and Jack Bruce were prior to Cream's formation in 1966. They were the rhythm section for a jazz-flavored London R&B band known as the Graham Bond Organization, fronted by Bond, a singer-organist who essentially fronted a band similar to a jazz organ trio, with the addition of tenor sax man Dick Heckstall-Smith.
Bond never toured the US with this unit, but the group had a following in England and are rightly considered part of the British Blues movement along with Clapton, Manfred Mann, Mayall, Georgie Fame, Rod Stewart, the Yardbirds and so on. Their mid 60's albums, however, offer some insight into the roots of Cream.
1964: "Camels And Elephants," showcasting Baker, from the album shown in the video, only ever released in England.
August 7, 1965: Onstage playing "Hoochie Koochie Man" Bruce is playing a Fender six-string bass.
Jack Bruce wrote "Train Time," showcasing his vocals and harmonica, Baker pounding away behind him (and Clapton laying out). The song's on the 1968 Wheels Of Fire album and they played it on their final tour that year.
1966: Cream's performance on the BBC's Top Of The Pops radio program.
1965: Bruce and Baker's original performance of "Train Time" with Bond.