LONDON — “What’s the worst thing you’ve seen on the internet?”

That question flashes up on the audience’s cellphones during “The Believers Are But Brothers,” an extraordinary, teched-up show by the British theatermaker Javaad Alipoor concerning the online radicalization of young men.

Mixing documentary theater, storytelling and performance art, “The Believers Are But Brothers” shines a light into the more abyssal recesses of the internet, where Mr. Alipoor has seen some truly terrible things.

The most striking feature of the show, which is written and performed by Mr. Alipoor, is that it invites its audience to join a WhatsApp group for its duration. At first this feels like a novelty — there are cat memes — but it also breaks down the usual hierarchy of a theater space, with the audience encouraged to make sassy remarks back to the performer.

Then it becomes a lecture tool, with Pepe the Frog cartoons used to illustrate a crash course on the 2014 Gamergate controversy, a conspiracy theory-fueled attack on women in the computer game industry; the alt-right movement; and that movement’s favored message board, 4chan. Eventually, the play becomes something dark and upsetting, as the WhatsApp messages start to ape the violent, posturing misanthropy of the Islamic extremists whose social media accounts Mr. Alipoor trawled researching the show.