Star Wars may be worth at least $4 billion to Walt Disney and Star Trek the subject of 12 mostly successful films and six television series for Viacom, but neither beats Doctor Who, the BBC adventure series about a wandering Time Lord that has thrilled audiences for five decades. The Guinness Book of World Records honored the show in 2009 as the most successful sci-fi series of all time.

Is that really fair? Taste is subjective, after all, and each series has seen more than its share of wins. For Fool contributor Tim Beyers, what makes Doctor Who so interesting is how prevalent "Whovians," as fans of the show are called, tend to be at pop culture events.

They came out in force at Denver Comic-Con, Tim says in the following video, overwhelming a conference room just for a chance to see Colin Baker and Daphne Ashbrook. In the 1980s, Baker succeeded Peter Davison as the sixth incarnation of The Doctor. Ashbrook appeared in a 1996 movie playing The Doctor's human traveling companion. That they both still attract so many fans speaks to the longevity of the franchise.

On a worldwide basis, more than 77 million in 48 countries watch The Doctor regularly, Reuters reports. Expect even more this November, when a 50th anniversary special airs. Neither the Starks and Lannisters of Game of Thrones nor the zombies of The Walking Dead have so large or fervent a fan base.

Why should this matter to you as an investor? Because passion can be a powerful profit driver. Look at AMC Networks (AMCX 3.42%). Thanks to The Walking Dead, which debuted on the network in October 2010, revenue growth accelerated from 10.1% in 2011 to 13.9% last year, according to data supplied by S&P Capital IQ.

For Doctor Who, the difficulty for American investors is that there is no way to invest in the BBC directly. Yet there are alternatives: Amazon.com and Apple (AAPL -0.57%), principally.

Both companies sell single episodes and full seasons of The Doctor's adventures. Amazon, like Netflix, also streams prior seasons, while Apple says iTunes executes some 800,000 TV downloads daily. Parts 1 and 2 of the latest season rank in the top 10 of iTunes downloads in Science Fiction and Fantasy.

Investing in Apple means getting a cut of the profits from those downloads. Amazon offers a similar opportunity. Will you take it? Please watch the video for Tim's full take, and then leave a comment to let us know which shows you're downloading, and from which service.

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