The song was about drummer Keith Moon's drinking problems. This would be the first of two songs from The Who written about Keith Moon, the second being "Doctor Jimmy" from the album Quadrophenia. Who biographer John Atkins calls it "a macabre tribute to Keith Moon."

"Dr. Jekyll and Mr. Hyde" has been compared to a Hammer horror film. The lyrics describe the good and evil elements within a single character, as in Robert Louis Stevenson's "Dr. Jekyll and Mr. Hyde" story. The music incorporates a "scarey opening" and has a melody led by Entwistle's bass guitar line, which Chris Charlesworth describes as "menacing" and Atkins describes as "grinding." It also contains a French horn solo that Charlesworth describes as "spooky." Atkins describes the melody as being "strongly inventive."

"Dr. Jekyll and Mr. Hyde" had been considered as a possible single release, along with "Call Me Lightning," but it was released as the B-side of "Call Me Lightning" instead. Atkins laments this decision, stating that although its horror film imagery was not ideal for a single, it was far better than "Call Me Lightning." He considers it one of Entwistle's best songs, saying that the "music and performance combine to create a perfectly chilling horror-comic Gothic mood piece." Charlesworth states that the song "succeeds admirably."

Two very different versions of this song exist. The first one, running 2:24, is the B-Side to the US single "Call Me Lightning". It is still available on the 1968 compilation album Magic Bus: The Who on Tour. The second version, which exceeds the former's length by 14 seconds, was the B-Side to the UK single "Magic Bus". This version has a more prominent guitar line, as well as spooky "Mr. Hyde" effects (the voice John Entwistle had used in chorus of the song "Boris the Spider") and can be found on the Japanese release of the Who's Missing/Two's Missing compilation released in 2011.

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